Help-I had sex with my friend and now my mom is on to us!?

My mom already found out I had sex with my boyfriend (an ex now) and that was pretty bad, but she wasn't too upset because she lost her virginity at 16 (I'm 17) and is generally not too conservative. Then I had sex with my best friend (a girl) and figured that if she ever found out it would be like the boyfriend all over again. But then we watched Black Swan and she FREAKED OUT at the lesbian scene and went on a rant about how sick, disgusting, sinful, etc. it was to be with another woman. I've known my best friend forever and I love her so much, and I don't see anything wrong with being with her sexually. We never act touchy in front of her, but one time she nearly walked in on us giving eachother oral sex and she didn't see anything but I'm afraid she might have heard. If she finds out, she'll never let my friend over again and she'll think I'm a pervert. She already kind of does but it would make it so much worse. My friend and I are getting an apartament together when we go to college (this fall, we're seniors) but if she finds out she'll never help me get an apartment or help me pay while I look for a job. But I can't end this thing with my friend. I trully love her and have never felt closer to any human being. What should I do?! And what should I do if my mom confronts me??