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 (let alone a whole raft of shit, which you know if you've been reading my comment threads).
There are really two basic reasons to get involved with anyone—for temporary fun, or to get into a relationship. If you're out looking for someone to pass a little naked time with, well, supposedly monogamously committed men (or women, or whoever—I'm using male pronouns here but feel free to sub in the pronoun of your choice) are less likely to be looking to settle down with you most of the time, but they're high-maintenance fucks. You'll never be able to go to his place, he's likely to have trouble paying for things (since many married couples have some co-mingled finances), he'll have to sneak around to see you and he'll probably rarely, if ever, be able to stay particularly late or sleepover. Plus, if he's just fucking around, he's obviously courting drama at home and, if he gets caught, he'll totally throw you under the bus even if his significant other doesn't find your contact info and contact you. And, having been on the receiving end of that contact from a pissed-off wife (who thought I was boning her husband when, in fact, I was the unwelcome recipient of his attentions and otherwise involved), that's never going to be a fun conversation. Regardless, if your goal in getting involved with someone is to have a little fun with little drama, married (and supposedly monogamous) men are going to be drama from start to finish.
If you're looking for a relationship, well, the problem with getting involved with a cheater is even more clear: The man is already telling you by trying to get involved with you that he has commitment problems. Monogamy, after all, is a choice—and one he made—and by getting involved outside of his relationship, he's basically telling you that, at a minimum, his communication skills suck, and it seems smarter to him to sabotage his own life, set his current partner up for more than just the pain of a break-up and just engage in all the drama of an affair rather than have a conversation with his current partner that their relationship isn't working. And that's a kind interpretation—after all, he could just be a guy who gets off on the power trip of fucking around on his partner, or he could have made a commitment he never intended on keeping, or he could just be a pathological liar. Whatever his personal pathology, if your goal is to find a partner with whom you can have a solid relationship—monogamous or otherwise—getting involved with someone to weak to leave a bad relationship with less than solid communications skills is probably setting yourself up for failure even if he leaves... and, quite often, he won't.
And yes, I am perfectly well aware that two people can fall in love—real love—even when one of them is otherwise committed. The question is, of course, what the committed person chooses to do about those emotions: he or she has to decide whether the existence of them is worth ending the pre-existing relationship over, or whether the existence of them does not and will never trump the pre-existing relationship. But you're not going to ever figure that out in bed together, and creating the golden parachute of a new relationship to cushion the end of the old is actually not a comfortable way to cushion a fall—gold's pretty heavy, after all.
(One caveat: I do acknowledge the existence of people who get off on the power trip of fucking other people's partners. Most people who end up the third party in a relationship aren't doing it maliciously or because they are turned on by the thought of screwing up other people's lives—but, certainly, they do exist. I realize that the few people who view sex or relationships as a conquest and a committed person as a harder target to obtain are, indeed, pretty fucked up people and, in my experience, pretty easy to spot. The woman, for instance, who snagged one of my hook-ups' cell phone while we were hooking up in a bar to program her number into it just to prove she could fuck him too is a pretty good example of how they are easy to spot. And so even if someone was actively pursuing your partner to "win," he or she still chose to allow them to "win"—and it says some about the person who was committed to you that they found that attractive.)
At the end of the day, though, I can say all I want about why it's a bad idea. Nina Simone sang it far better than me years ago—and without slut-shaming, either.
[Image via Professor Bop on Flickr, Creative Commons Licensed]