I caught my teenage daughter having sex with another girl?

On Saturday night my daughter had one of her friends spend the night and of course, I expected nothing out of the ordinary because she and her friend have sleepovers a lot and her friend is a sweet girl from a nice family. Anyway, I am a very light sleeper and last night I woke up at around 3:00 am and went to the bathroom. When I was walking back to my room, I walked past my daughters room and I heard soft moaning noises, and saw that the light was on. I wanted to see what was going on, so I cracked open the door a little bit and saw both my daughter and her friend on the floor, completely naked, and my daughter's head was between the girls legs and was performing oral sex on her. They didn't see me at first because the friend had her eyes closed and was moaning, and my daughter was too busy...well...you know. I didn't know really how to react because I was so in shock, but I yelled at them to get up and get dressed. The next morning, I made the girls parents come pick her up early and I explained to them what happened and that I do not want our daughters seeing each other anytime soon. I'm also going to put my daughter in therapy. My daughter had never mentioned to me that she was a lesbian or bisexual, because she's had boyfriends before and has a lot of posters of boys in her room and such. But when I questioned her about her sexuality after her friend had left, she admitted to me that she was bisexual and has been for a long time. How is that even possible? And, my daughter and this girl are best friends! Is it normal for teenage girls who are supposedly "just friends" to engage in sexual activities together?