He's a single guy. She's a happily married woman. They are about the same age (late 20s/early 30s). They meet through work (don't work in the same place), talk a lot (phone, texts, IMs, etc), are very good friends, then one day they kiss and it gets to be a little more than just friends and they're trying to stop but it keeps coming back on somehow, no matter how much they try to separate and not keep contact etc. Whats the way forward and is this common?