1) The short, correct legal answer

Adultery (or sexual infidelity) is a ground for legal separation in the Philippines.
Adultery is not a ground for annulment (voidable marriage) and is not a direct ground for a declaration of nullity (void marriage).
A spouse’s adultery may be relevant to other remedies (criminal prosecution, damages, property and custody consequences, or as evidence supporting a different theory like psychological incapacity), but it does not, by itself, make the marriage void or voidable.

To understand why, it helps to distinguish three different family-law cases that people often lump together as “annulment.”

2) Key concepts: annulment vs declaration of nullity vs legal separation

A. Annulment (voidable marriages)

An annulment applies to a marriage that is valid at the start but can be voided because of defects recognized by law. These defects are limited to specific grounds under the Family Code (commonly: lack of parental consent for certain ages, insanity, fraud of specific kinds, force/intimidation, impotence, serious and incurable STD existing at marriage).

Important: These grounds focus on problems existing at the time of the wedding (or at least traceable to that time).

B. Declaration of Nullity (void marriages)

A declaration of nullity applies to a marriage that is void from the beginning (as if it never validly existed). Grounds include lack of essential or formal requisites, bigamous marriages, incestuous marriages, marriages against public policy, and psychological incapacity (Family Code Article 36), among others.

C. Legal Separation

Legal separation is different: it does not end the marriage. It allows spouses to live separately, with consequences on property, custody, support, and inheritance rights, but the spouses remain married and cannot remarry.

Legal separation is the remedy the law provides for certain serious marital offenses that occur during the marriage, including sexual infidelity.

3) Where adultery fits in Philippine law

A. Adultery as a civil ground: “sexual infidelity” under legal separation

Under the Family Code, legal separation may be granted on enumerated grounds, and one of the primary grounds is sexual infidelity (commonly understood to include adultery-type conduct).

What this means in practice

A spouse may file a petition for legal separation based on the other spouse’s sexual infidelity, without needing to twist the facts into an “annulment” theory.
Legal separation is the proper civil remedy when the core complaint is post-marriage cheating.

B. Adultery is not a ground for annulment

A spouse’s adultery after the wedding does not match any of the exclusive annulment grounds for voidable marriages. Annulment is not a “fault-based” dissolution remedy in the way people often assume.

C. Adultery is not a direct ground for declaring a marriage void

Cheating does not automatically mean the marriage was void from the start.

However: adultery can sometimes appear in cases filed on other grounds (especially psychological incapacity), but in that situation:

The legal theory is not “adultery = void,” but rather
“There is a qualifying psychological incapacity that existed at the time of marriage; adultery is a manifestation of that incapacity.”

Courts generally require more than proof of infidelity—there must be proof of a legally recognized psychological condition meeting strict standards, and it must be tied to the time the marriage was entered into.

4) The separate criminal case: adultery vs concubinage (Revised Penal Code)

Philippine law also treats certain extramarital relations as criminal offenses, distinct from civil cases:

Adultery is traditionally the crime charged against a wife who has sexual intercourse with a man not her husband, and the male partner may also be liable if he knows she is married.
Concubinage is traditionally the crime charged against a husband under narrower conditions (e.g., keeping a mistress in the conjugal dwelling, cohabiting elsewhere under scandalous circumstances, or having sexual intercourse under certain aggravating circumstances).

Civil vs criminal:

You can pursue legal separation even without filing a criminal case.
A criminal case can exist even without a legal separation case.
A criminal conviction is not always required to establish sexual infidelity for the civil remedy, but the civil court will still require competent evidence.

5) What legal separation based on sexual infidelity does (and does not) do

What it does

If granted, legal separation typically results in:

Right to live separately (formal separation of bed and board).
Separation of property: the property regime is affected; the “offending spouse” can suffer disadvantages depending on the property setup and the court’s orders.
Custody considerations: the child’s best interest remains the controlling standard, but fault can affect custody arrangements in appropriate cases.
Inheritance and benefits consequences: the offending spouse may lose certain rights (e.g., to inherit from the innocent spouse under certain conditions), and beneficiary designations may be affected depending on the benefit and governing rules.
Support: support issues are addressed as part of the case, subject to law and equities.

What it does not do

It does not dissolve the marriage.
It does not allow remarriage.
It is not the same as annulment/nullity, and it is not a “divorce substitute” that frees parties to marry again.

6) Procedural and timing rules specific to legal separation (practical and critical)

Legal separation is tightly regulated because the State has an interest in preserving marriage.

A. Filing period (prescription)

A petition for legal separation must be filed within a limited time from the occurrence of the ground (commonly discussed as a five-year window in the Family Code framework). Delay can bar the action even if the infidelity is real.

B. Bars and defenses (why a strong case can still be dismissed)

Even with proof of sexual infidelity, legal separation may be denied or dismissed if any of these apply:

Condonation: forgiveness (express or implied) after learning of the offense.
Consent: the complaining spouse agreed to or permitted the conduct.
Connivance: the complaining spouse facilitated or orchestrated the infidelity.
Collusion: both spouses cooperate to fabricate grounds to obtain legal separation.
Reconciliation: if spouses reconcile during the proceedings, the case generally cannot proceed.

C. “Cooling-off” and reconciliation efforts

The law requires a period intended to encourage reconciliation and prevent impulsive filings. Courts also involve the prosecutor to ensure there is no collusion.

D. Evidence expectations

Courts typically look for credible proof—this may include testimony, admissions, documents, messages, photos, hotel records, pregnancy/paternity-related facts, or other corroboration. Mere suspicion, rumor, or private belief is usually insufficient.

7) Can adultery help support an annulment or nullity case anyway?

A. Psychological incapacity (Article 36) — where cheating is often mentioned, but not enough by itself

Some petitions for nullity allege psychological incapacity, and the narrative often includes repeated infidelity. The key point:

Infidelity is not the legal ground.
It may be treated as symptom or evidence of deeper incapacity.
The incapacity must be juridically relevant and shown to have existed at the time of the marriage, not merely developed later.

In short: adultery can be part of the story, but it is rarely the whole case.

B. Fraud (annulment ground) — usually not “cheating”

“Fraud” as an annulment ground is not a general “deception” category that automatically includes being unfaithful. It is limited to specific kinds recognized by law (e.g., concealment of pregnancy by another man at the time of marriage, concealment of a serious communicable STD, and other narrowly treated deceptions). Post-marriage adultery does not fit.

8) Related remedies and consequences beyond legal separation

A. Damages and civil claims

Depending on the facts, an aggrieved spouse may explore damages under general civil-law principles (e.g., emotional distress-type claims), but Philippine courts are cautious; outcomes depend heavily on the specific cause of action pleaded and proof presented.

B. Protection orders / violence-related remedies

If the infidelity is accompanied by abuse, coercion, threats, or economic control, separate protective and criminal remedies may apply. These are independent of legal separation/annulment/nullity.

C. Property protection while the case is pending

Courts can issue provisional orders to protect assets, ensure support, and stabilize custody arrangements while a legal separation case is ongoing.

9) Common misconceptions (Philippine context)

Misconception 1: “Cheating is automatic annulment.”

No. Cheating is handled primarily through legal separation (civil fault remedy) and/or criminal prosecution, not annulment.

Misconception 2: “If I win legal separation, I can remarry.”

No. Legal separation does not restore capacity to remarry.

Misconception 3: “I need a criminal conviction for adultery before I can file legal separation.”

Not necessarily. Legal separation is a civil case with its own standards of proof and procedures.

Misconception 4: “Annulment and declaration of nullity are the same.”

They are different remedies for different kinds of marriages (voidable vs void), with different grounds and effects.

10) Bottom line

Adultery (sexual infidelity) is a recognized ground for legal separation in the Philippines.
Adultery is not, by itself, a ground for annulment or declaration of nullity.
If the objective is to end the marriage and remarry, legal separation does not accomplish that; only a successful annulment (voidable) or declaration of nullity (void) does—on their specific legal grounds, not merely on proof of cheating.