To have sexual intercourse before you are "married",or even after you may have been divorced.
(is a sin)
Basically when your not married and you have sex, that is a sin as far as Roman Catholics are concerned.
If you have sexual intercourse while your married, it would then mean you committed a sin called
Not a thing I would be proud of "adultery?"
But I've never been married or cheated.
As hard as it may be to believe.
It's all true.