DEAR AMY: Recently I received a Facebook friend request from “Frank,” a man I had an affair with 20 years ago. This was more than an affair because Frank and I were best friends. We talked about everything! I would have left my then-boyfriend for him back then, as that relationship was going south fast. Frank and I both ended up with other people and lost contact. Frank tells me that his relationship with his wife is like they’re roommates and that he is seeing someone on the side, but would like to “catch up.” I’d love to do this because he started talking about our past and our amazing sexua…