15 year olds do experiment sexually - and those that are curious about same sex attractions may do so because there is no threat of pregnancy and it is easier to cover up the sexual aspect of the relationship. How far you want this to go and how you want to exercise parental authority and guidance is up to you.
I think it is good to let her know that you know - this way she knows that sneaking does not work. And I think JeanLynn's advice about stds is very good.
It's important to facilitate open communication with her so that she does not shut you out, and to keep the love between the two of you open.
I don't remember this kind of experimentation being common when I was a teen in the 1970s - but we did not have so much sexually charged images surrounding us and as easily accessible as todays kids, so probably the idea to try some of these things never occurred to a lot of kids back then.
I don't know how this is for teenage girls today, but my son started a GSA club at his high school as a straight boy who wanted to encourage gay and bisexual friends to come out in a safe social atmosphere. He did this to honor my brother, his uncle whom he barely knew because the man died of aids as a closeted homosexual. My son found that the boys who joined the group in high school proclaiming themselves to be bisexual were actually more homosexual the older they got as the college years went on. I don't know what the experience was for the girls.
If you believe that your daughter may be gay or bisexual, please get in touch with
PFLAG (Parents and Friends of Lesbians and Gays). This organization was very valuable for a friend of mine when her 20 something daughter came out. They offer a lot of guidance which may be of help to you.