Ok, I'm in a dilemma and I'd love some help with this one.

I have two friends (well, we don't see each other anymore but I have known them forever) and they have both been in a relationship with two girls and I've always gotten on with their GF's (Let's call the girls A & B). When I say well, I mean VERY well. I usually feel awkward for some reason around girls, but with these two they don't make me feel awkward.

Anyway, 'A' split up with her man and was comforting me through text messages after I had some bad problems of my own (bear in mind that I hadn't spoke to her in months as me and her guy weren't getting along). Next thing you know we're at hers, there's huge sexual tension between us, I make a move after she is putting every sign under the sun out there, and she was all for it. We end up meeting a few times, just making out & chilling, no sex ("I'm on my period" such a great excuse) then it fizzles out and we go our own ways. But she will still text me, and I have bumped into her before and again the signs were ALL there. But I never made a move. We still text now & then.

'B' - I saw her a few days ago. She has split from her man (another old friend who I haven't seen in years) and we were talking and her body language was confirming what I thought. I could tell her heart was racing, as when she talked she was talking the words were all over the place, as was her breathing. Then when I started talking to her about her and her son, she was ALL smiles, dilated eyes. Then when she said how her man did nothing for her (like ask her how work was/show attention) and I told her what I'd do, she was again all smiles/big eyes etc.

But here are my questions:

Why do they seem to feel alright hitting on me?

What is it about me that makes them go kinda weak when it's just us two? (I can read the signs)

Will it ever go further than what I've outlined I've done with the one above? And about "B", if I wanted her, how do I approach her?

I'd just love to know why they choose me when they have zero interest in any other mutual friend.

Sorry about the essay! It's my first time here.