Girlfriends was about showing how "regular" girls could still fall in love with another girl, this was all about self-discovery and accepting those feelings. Reason why Mari and Akko were the only "lesbians", because if the whole cast were, it would have lessen the impact. You can't make a girl agonizing about whether she can fall in love with another girl if every other girls around are dating each other.
And Mari didn't reject the kiss from Tamamin because "eeew, it's a girl !" she did because she was already in love with Akko who was there watching. And the way it played out, it really felt like the trigger for the rejection was Akko being there. If it had just been the same group minus Akko, she might not have rejected it, at least not so violentely. Truth is, Neither Akko or Mari ever considered other girls seriously. Even guys, it was never really stated that they were into guys at some point. Akko got drunked and let some random guy take her to an hotel without really realizing it and if she was annoyed at still being a virgin afterward, it wasn't because she wanted to have sex with the guy, but because she felt offended that a guy could see her naked and not managed to "get it up". As for Mari, it was just a way to try and run away from her feelings toward Akko. She couldn't even kiss him, when he tried, she ran away.