First of all, do you know that your daughter is living a “gay lifestyle”. It can’t be assumed. Even if she is, you need to separate that from the idea that she is attracted to women, which is a situation she found herself in, not one she chose. It has nothing to do with you.
Consider that she likely knew how upset you would be, and has also very likely struggled with it herself before telling you. She thought it was important for you to know this because she wants you to continue to be close, and this looms quite large in her life right now. It likely would have been easier for her to say nothing. You are now in a position to be a great Catholic resource to her in how to deal with her situation, if she asks for your help. Not if you alienate her however.
I think it is unlikely that she is just experimenting. Girls who do this don’t usually identify as gay, and don’t tell their Catholic moms about it unless the hate them. There is good evidence that homosexuality is related to differences in brain structure in many cases, and if that is the case with your daughter she is unable to change that, even if she wants to.
Remember that many Catholics make it very difficult for homosexuals to live a Catholic life, and be conscious of not letting this happen to her. Catholic homosexuals can feel very alone and rejected, even when they are living a celibate life - celibacy in general can lead to loneliness. Some leave the Church because they cannot deal with the isolation.
If your daughter is living a participating in homosexual activity, I would be inclined to treat it just as if she were involved in an unmarried heterosexual relationship. You can tell her you think she shouldn’t, but don’t harp on it, it won’t convince her. Remember that young people often have to experience things before they really work through them - many kids try promiscuity before they realize why it is a problem, or partying before they realize drunkenness is bad. Even if they know it at an intellectual level before, it often takes more to know it in your bones. She will mature and change over the years.
In the end, if she decides on living her whole life in a way you don’t agree with, that’s what free will is about, and part of what parenthood is about. It not very likely she is trying to be, an immoral person, she is trying to figure things out like the rest of us.
