I have been in this exact situation, the only difference was, we spoke about it FIRST.
I personaly don't consider it cheating, however by definition it is.
We came to an arrangement where she could do anything she wanted with another girl simply because it didn't bother me.
Guys that have problems with that I think are insecure, all jealousy etc.. comes from being afraid you will loose the girl or guy to the other person, or selfishness with your time.
Either way, its not a good personality trait.
If you wanna make up, you better have a long conversation with him about why he doesn't like it..
As for common, *most of my girlfriends tend to be bi, and I like the fact that they eat pussy, especialy if they invite me to a few munches too
