In my case, he and I tried to resist for ages - the attraction positively sizzled. others thought we were having an affair long before we actaully had one - and ironically, once we were having the affiar, the tension between us reduced.
We did try to warn each other off - but that somehow only made the affair even more inevitable.
I don't avtaully regert it - I did truly love him and I beleive he loved me. But at the end of the day, we mutually decieded that there were other people to take into consideration (his wife and a baby who had already been from pillar to post). I'm not saying the break-up was easy - and seeing them socially afterwards (as his wife was now a good friend) was hell for a long time.
If I think about, even now, 20 years later there is a teeny tinge of sadness in my relationship with him today - and we both make sure never to be in too intimate a surrounding or to talk "feelings" too much.