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> Of course, having sex with someone you aren't married to is a sin...so there
> you are.

Let's see.  First of all, what's the meaning of the word?  A sin is an act that
hurts your own or someone else's dignity.  Basically, everyone's conscience
tells him/her which kind of acts are sins and which are not.  However, some
people don't trust their conscience or are trying to refine its power of
judgment.  For these people, religions provide catalogues of sins, or
catalogues of do's and don't's, e.g. the Ten Commandments.  These catalogues
are supposed to help you with detecting and avoiding bad behaviour (or sins),
but they cannot completely replace your conscience.

Back to your example.  If you have had sex with a woman you aren't married to
and  a) she feels remorse for having done so,  b) you feel remorse for having
done so (because you have been unfaithful to your wife), or  c) your wife is
angry with you because you have been unfaithful, then this has been a sin.
However, if everybody is happy and will never feel remorse, then this hasn't
been a sin.