Addressing your paranoia of thinking the girl you like is already taken:
When you like a girl, ask your best friend if they know anything about that, I.E, whether they have a boyfriend/girlfriend (you never know). Because, maybe she IS taken, but then you still have a friend. And if the relationship goes south, you've still got a chance, instead of having ruined it by going absolutely bonkers when you see her hanging out with some other guy.
A suggestion: Use facebook, if you've got it. It says right on their profile page whether they are seeing someone or not. And a word for the wise: Sometimes girls say they are "married" to other girls via facebook. That's all just for fun, if you didn't know.
Also, don't be so defensive if they tell you they don't think you're boyfriend material. Ask them why? Don't pressure them into telling you though.
Scenario:
Girl:...I just don't think you're boyfriend material.
You: If you don't mind me asking, can you tell me why?
Don't attack automatically. Ask questions, because if they're friends, they'll tell you.
Lastly, on this subject, if they tell you they don't think you're boyfriend material and you don't attack, tell them that you think friendship is the most important thing, and you'd still like to be friends. This way, she doesn't assume you still want her everytime you say hi to her or ask to hang out with her, etc.
As for your lesbian friend. I bet you felt awkward about that one. Anyways, she took it as an attack and you fought.
What you absolutely HAVE to do, if you still want to be friends, is to apologize. IMMEDIATELY. Tell her you were wrong, an asshole, etc., and how you were only joking and you see how it was a bad idea, because girls get defensive over everything. If they think something is attacking them, trying to lower their self-esteem, etc. etc., they will go full out bitch mode and...it isn't pretty. All girls have feelings, no matter what age, or if they act skater, punk, emo, goth, tom-boy, etc.
Don't even ask.