Cas is there anything else going on in your brain except getting opinion on your seedy love-life? You are cheap, the married man is a cheating b*stard and his wife and your boyfriend deserve better. In reply to your wider question, no i wouldn't and I haven't. If I liked someone enough to cheat I would leave my partner first.
Ive had plenty of opportunities but have no interest in cheating. the way i see is that you obviously dont care for some1 if you'll cheat on em so you might as well not be with. to do otherwise is JUST malicious!
If I was totally in love with the person I would go for it and maybe you never know until you are in that situation I would go with them even if they were taken but only if I really liked them. But If I was seeing someone that would be because I wanted to be with them so I wouldnt be falling for anyone else anyway.
cas23, try using a few of the remaining brain cells and try and focus on something other that betraying ones partner, it doesn't occupy most peoples minds like it does yours, getting a bit boring now ZZZZZZZZZZZZZZZZ!!