Anonymous wrote:I would definitely not assume that she is into the idea of having a fling with you, just because she is coming alone. Plenty of people go to high school reunions without their spouses not planning to have a fling. Others, married or single, may be looking to kindle or rekindle something -- but I don't think it's that often that they're looking for a one-nighter.
+1. I tend to go to those things by myself because DH doesn't know anyone there and it would be super-boring for him, but I love him and have zero desire to cheat. If someone from the past propositioned me, it would go nowhere.
Unless you guys have been flirting via that Facebook friendship or you have some other indication she is interested, you have a great chance of making a fool of yourself.
I am not going to get into morality of adultery since it wasn't what you asked about and other posters covered it anyway.