There are many reasons not to get involved with someone who is otherwise monogamously committed to another person. It's not because you're a slut destined to ever be unhappy, or because you're betraying some sisterhoodly duty to prevent someone else's man-child from betraying her. It is, however, because, regardless of what your relationship or dating goals are, you're likely setting yourself up for failure (and a whole raft of shit, which you know if you've been reading comment thread).
The person with whom you have decided to have a monogamous relationship owes you fidelity, as you owe that person the fidelity you also promised. If that person breaks that promise, I can guarantee one thing: he or she made the choice to do so.