I was initially upset but now I see it as just another example of her wanting me to be something that I'm not.
I think teens would react this way to all sorts of things. You grow up with an image of who your parents are, and it shakes you up if that changes. It would probably be similar if she'd just learned that you wanted to spend winters in South America; or take up the accordion; or join a motorcycle club; or become a talk show host (assuming none of these fit in with your character before). I was about 19 when my mom decided to rent an apartment where she could spend time alone, and to join a new age drumming group. I felt like she betrayed my image of who she was supposed to be -at home making dinner for dad. But behind it all, daughters do want their moms to be happy. She'll come around.
As for the lesbian bit -no one likes to think of their parent having sex with anyone anyway! Grooossss! Again, I bet she'll come around. Especially if she accepts it in her friends.