I'll comment to give you a little more of the masculine perspective.
OohTeenyWahine, says that if he truly cared for you he would realize how much his opinions impact you as well as how much Theta means to you . . .
I would switch this around and say that if he was truly mature he would realize this. His caring about you doesn't obligate him not to express his opinion or share yours about something.
Ladies, never use the "if you cared about me, or if you loved me you would/would not do something." The guy may give in but part of him will actually HATE you for using that kind of trump card. And we men tend to carry grudges. Men are brought up to believe in control and to be obviouly forced into something by emotional blackmail is not something we swallow easily. Its as bad as threatening the relationship, I'll break up with you if . . .
So don't use that agument
not that OohTeenyWahine was suggesting you should.
I believe 33girl hit it right on the head when she said to bring him around your friends. Also bring him around other Greek that you know outside your group. Invite him to philanthropy events. If he demurrs or gives you the standard silly Greek thing line, give him your best quiet puzzled look and ask him, "well if you don't want to help starving children, or don't have time that's fine, but please don't use the reason that you won't help because Theta's are trying to help."
I would feel abashed and help if someone said that to me . . . and I'm kind of unfeeling.
Don't mention your sorority or Greeks per se in conversation, refer to them as friends or a group etc. He might have more of a problem with the word, it hits a trigger in his psyche. After all he doesn't have a genuine dislike . .. he's dating you
.
Just remember to not argue, just ask specific and pointed questions.
Oh you don't like my friends?
What did Fred or Jeff or Stacey do personally and specifically to you?
Nothing? So you just dislike them and talk badly about them for fun? That's fine, can we change the subject?
Remember, no one in your realm of acquaintances has done him harm. So its his hang up and he doesn't have the right (as was pointed out earlier) to inflict you with his hang-ups. But if he's kind of immature he'll do it anyway
got to love us men
.
Well good night and take care . . ..