What do you call someone who betrays his/her spouse sexually? Is the word different for men and women? Is it different for people who are in a relationship and not still married?
For a word that can refer to both married people or people in a relationship, you can use cheat, cheater, two-time (informal). Two-time is also used as verb, as reported from the NOAD: He was two-timing a fiancé back in England.
For people who are married, you can use adulterer (which is gender-neutral) or adulteress (when referring to her).
The term philanderer could be used, although it is more rare and often used along with the other terms mentioned (adulterer/adulteress/two-timer/cheater). The person who engages in philandering is a person who engages in at least one, if not many frivolous sexual exploits outside of the bounds of marriage. Philanderer has a strong male connotation. I have not heard this term used in reference to females.
You dirty, rotten, two-timing, cheating, adulterous philanderer! I trusted you!
When referring to an unfaithful woman, sometimes much more vulgar terms are used, usually in relation to her sexual morals and/or her potential willingness to engage in prostitution.
Cheating on one's spouse is not the only way to betray them; there are several others e.g. disclosing a spouse's deepest secrets, causing your spouse some form of public humiliation, etc.
If the betrayal involves sexual escapades or an illicit/extramarital affair, then relevant terms are:
cheat • womanizer • skirt-chaser • adulterer[ress] • fornicator • heartbreaker • philanderer • husband-snatcher • two-timer • unfaithful spouse • ladies' man • seductress • Don Juan • Casanova
While English does retain some nouns for a person who displays infidelity in a marital relationship, these terms have the sense of being curiously dated. To call a person "a cheat" or "a philanderer" harkens back to the kind of language you'd hear in some Doris Day-Rock Hudson comedy from yesteryear.
It is possible to say, "he's an adulterer," but to do so is branding the person with an identity, and socially it's becoming less permissible to do that. Rather, the more common ways to express this sort of thing is through more active terms that describe the activity itself:
- He's cheating on her.
- She's seeing another man.
- He was unfaithful to her.
- She went around behind his back.
From a British perspective, I would say that cheat (exactly the same spelling as the verb) and two-timer (with an r) are the standard nouns describing people outside a marriage. Cheater is used colloquially, but is frowned upon as incorrect. Two-time is used as a verb, not a noun.
As others have stated, adulterer and adulteress are the standard, formal terms for people within a marriage.
Works for both man and woman:
Bedswerver One who swerves from and is unfaithful to the marriage vow.
It is infidelity or an affair (marriage) vs cheating "the game" and its "players" and "sluts" on both sides, when it comes to dating. Most marriages are just a pipe dream between two "players" who regain their love for "the game" eventually. Well, on average it takes seven years; Then the carriage turns back into a pumpkin.