c) he doesn't consider her a threat - this one I put in here because guys can sometimes...hmmm...view their opponents as level of threats. If you were clear about exactly how you feel, he could very well be seeing it as a fling or lesbian love he doesn't need to be concerned with
Guys, and girls actually so this isn't gender specific, will look at other partners almost like a threat level. Simply consider the One Penis Policy. Lots of guys believe women on women relationships are less of a threat than a man on woman relationship. However if you were involved with a guy, the threat level would be different, forcing his hand to become more alpha and strut...
This really simplifies it, btw, but I read about it often. Heck on one forum I am on there is a group of guys defending the OPP to death believing, truly, in their hearts that female relationships are more fleeting and less important than "normal" ones. This of course is a massive slap in the face of any lesbian, bi-sexual or other relationships.