My brother-in-law cheated on my sister, she then did the same(I have no clue why.
Makes me think of couples with an 'open marriage'. The idea generally is that if both parties are okay BEFOREHAND with the idea of their partner 'sleeping around', it can still work.
I'd probably be okay with such a thing too, if agreed to BEFOREHAND. (And by 'before hand,' I mean 'before the marriage'. Discussing having an open or closed marriage after the wedding is just bad form and probably would break trust - if you want an open marriage, make sure your partner does to before you get wed)
The important thing to remember, IMHO, is that marriage consists of more than just sex. It is a lifelong relationship/partnership built on a foundation of trust. Having sex with someone outside the marriage DOES NOT necessarily 'break' the marriage. Doing so WHEN THE OTHER PARTY EXPECTS FIDELITY and TRUSTS you to maintain it *does* break the marriage.
A fanatic is one who, upon losing sight of his goals, redoubles his efforts.
--George Santayana