This is something I feel compelled to write about. Some of you may be offended, but I do not mean it that way. This is something I feel very strongly about, but it's still just my opinion...
I do not understand people who cheat on their spouses/ significant others. I say if you are that unhappy in your relationship either fix it, or end it. Don't complicate things even more by bringing in a third person. Tell me how bringing in another person in your relationship is going to make things better, for you or your relationship?
Now if you are single but seeing someone who is married/ with someone else, I have some questions for you too. If they will sneak around, lying and cheating on their partner, how the hell can you possibly believe that they wouldn't do it to you too? What kind of relationship is it going to be where it starts out with one or both lying and sneaking around? If it were me, I'd wonder if that person wasn't just using me as a stepping stone... until the next one came along.
I'll end this with something I heard someone say once. If you really think about it, it's SO true.
Just because it's different, doesn't mean it's better.