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 "three-date rule" is stupid. So's the five-date rule, the six-month rule and any other rule that someone's told you should govern the time in your relationship that you choose to engage in physical intimacy. The problem is that, in sorting through all the messages of when you should or should not submit to what the other person apparently wants from the get-go, not enough people get around to considering what feels right to them.