Oh man, I feel your pain.

There was a point for a while- about the time I was a freshman in college- when I was good friends with a lil' group of girls... let's call them A, B, and C (just as you did). All were good friends, but Girl A was the youngest, so they were very protective of her. B & C had boyfriends and were sexually active. I ended up taking Girl A's virginity and we had a secret little relationship. Well, then, one day, when the subject of sex came up when they were all hanging out, Girl A announced that she had been sleeping with me. Girls B & C were furious, and started accusing me of "seducing" and "taking advantage of the innocence of" Girl A. They also told Girl A she should be ashamed of herself for throwing away her virginity so frivolously. The thing was eventually forgotten after Girls B and C had their little hissy fit and eventually saw that they really had no right to speak, as they too had boyfriends and both had lost their virginities to them. (I eventually ended up sleeping with B and C, buuut, that's another story for another day)

My advice? Tell Girl B "Look babe, you know I love ya and you're my homegirl, but listen, you can't control my love life or girl A's love life. That's our decision and our decision alone- you'll always be my best friend, but let me have some fun, okay? You know I'm not a bad guy and you know she's not a bad gal, so what's the worst that could happen?"

She'll probably get over it soon. Good luck