What a great question! Since I read this book I wont give away spoilers, (BTW loved it!), but the best way to answer your question is simply to say 'it depends'. My rule is that the hero must not have started up a romance with the heroine. Once the romance is on all other sex partners are not acceptable. It's disturbing to see a hero you love get it on with someone other than the heroine, but in context of the story it can work. However, I read one book where the hero breaks it off with the heroine towards the end and finds himself having sex with a courtesan the next day. The heroine even walked in on him and took him back within an hour. (Yep, hated that book.)