I had sex with my guy best friend

My guy best friend is in the middle of a separation from his wife. It's a very long story but the short version is she fucked a her major(she WAS a marine) and got pregnant by him but aborted the baby. They're sperated but have started the divorce process. In our state you have to wait 3 months to divorce if you have kids. They have a 17 year old son who lives with him. It's been way over the 3 months but he's been going through things with his brother dying so he hasn't really been on that divorce. He just got his divorce lawyer. How this all happened was I took him out to celebrate being 40. He wanted to stay inside and be depressed so I took him out to celebrate. We got drunk and ended up having sex at my apartment. He was pretty upset when he found out. He was like "Oh my God what did we do? This can't happen again". Well... it happened again, and again. The last time it happened he said "How does this keep happening! Why do we keep doing this! We said we never do it again and we do it again". Finally I sat down and had a talk with him. About why we keep having sex and do we have feelings for each other and he said "No. Yes. Idk. I just don't want to cross that line. Me and my wife were good friends.... And look what love did to us.... Love ruins things and you're my best friend.... I think once we cross that line to where it's not just sex.... it's love.... Everything will change..." What is your guys opinions on what I should do?