(Edit: This isn't directed at anyone!
I am biased, because sex is not just sex. It means something to me, and has an emotional attachment. Would you like someone sleeping with your partner behind your back? How would it make you feel (if you didn't know about it beforehand/agree to it)? Do you plan on getting married, because vows would seem meaningless if you did, but slept with a married person.
Those are just a couple of the things I'd think about. I am very much a 'do unto others' person. To begin with, I would never do that to my partner. We are committed to each other. If things were in such a way that I felt the need to sleep with another person, then I'd break it off with my partner at the time.
What if you became emotionally attached? Or they did? Too much hassle and it could be asking for trouble. And for those who say 'what if nobody had to find out?'...I would know. And that would be the worst thing; having to live with it, and being the 'other' woman. But having said all that, each to their own. If you want to take the risk, it's your life
It might even work fo some people