Been down that road. Things can get complicated and messy with kids, etc. And if you are into a serious one (like I was), it may take a long time, but the spouses will find out. In our case we both had noting to lose in that regard - i.e. neither spouse was worth keeping. However - it still made for some tense moments at discovery - and you really do feel bad for the other involved. Also you can not predict what the other party you are involved with will do or not do - mine didn't hide anything - would say after a while she almost deliberately left things where they could be found by her spouse.
Having said that - I had an awesome relationship with this woman, sadly after that, most are hard to live up to her. However, we had a great run, almost 8 years, and I would not have wanted to miss that. - And we are still really close friends.
If your marriage is expendable, you have a right to be intimate with someone (something friends with good marriages can't quite grasp).
However, my advice is that if you are married to someone decent and worth it, Think twice.