I'm just going to put it out there. I have a whole lot of disrespect for people who cheat on their spouses, but for the most part, I don't fault those they cheat with.
It's one thing if a guy (or girl) is dogging the steps of a taken person trying to catch apples from the drunk tree. Another thing entirely if you're out one night and a pretty girl throws herself at you. And damn sure if I met a girl I thought I had a future with, I'd pursue it as hard as I could.
I just wouldn't let anything physical happen until she was unattached. Not a great starting off point to be 'the other guy', and how the hell do you ever completely trust somebody who couldn't do the right thing by somebody they allegedly cared about?
The onus of doing the right thing is on the people involved in the relationship.
Further to that, three of my relationships in life (one of them a wonderfully supportive two year relationship) started with me meeting the person while they were in a relationship. I never once felt bad for the guy who lost the girl. If he was doing his job, she wouldn't be up late talking to me.
I'd think more of a person if they did the mature thing and broke it off before they did something stupid (I mean, if sleeping with me isn't stupid, what is?) - but sometimes it's hard as hell to break up with somebody. Meeting somebody else who stirs butterflies in your stomach or makes you feel alive is sometimes the kick in the pants a person needs.