Gotcha!!! What was going on there?

When I was at a guy's house the other day, I knew he had a girlfriend, but for the life of me I couldn't have guessed that she would suddenly be standing in the doorway while we were fucking. But when she suddenly didn't make a fuss but said we shouldn't be disturbed, it made her horny, I was amazed. Well, it was fine with me, I'd been in the mood for a really hot threesome for a long time anyway. Especially since the guy fucked all two of us really well. How would your boyfriends have reacted?