I made general assumptions, i didnt view it from ur
perspective..well, i dont noe u well enuff .. 
In anyway, Fireice is right. Its abt communication. I got two real
life scenarios.
One is a friend of mine, abt the same age as me. His girlfriend
would meet new guys and go cheong with them. He is very ok with it,
in fact if they each met someone to go out to have fun with, they
would at least tell each other beforehand.
As for my second friend, he seriously fools around. Worse, if he
and his galfriend go out and drag me along sometimes, he can even
exchange glances with other gals right behind her back literally.
She knew his character, and at times she knew he really went out
with another gal, meaning havin an affair, or to even have sex. By
the way, he is very casual abt having casual sex.
He tried means and ways to hide it from her, but she knew. Quarrels
were always ensued until one fine day they broke off. But for long
time, she tolerated it, because she is really in love with
him.
U see, it all depend on how well u strive to understand and
communicate with ur partner. If u are one step overzealous, u might
lose a good guy, and if careless, u might end up having a
cassanovic burden.