Pre-marital sex OR ...?

[Swizz]

I am writing an english essay on sexual morality etc. and i was wondering what it is called when you have sex, outside of your marriage. Sex before marriage is pre-marital sex, but what is it called when you are effectively 'cheating on your wife/husband' with someone else? Thank you!

[amarillis]

One term for it is adultery, or can be referred to as having an affair

[woofgang]

extra marital sex ie sex outside marriage

[jcherbstritt]

...as opposed to extra marital sex when things are going well and you get alot of it!

[RebeccaSJ]

I believe the word you're looking for is extramarital.

[RebeccaSJ]

Adultery, an affair.  Or simply cheating.