I imagine it must have been very startling to hear your ten-year-old daughter describe herself as a lesbian and express interest in sex with other girls. I think that it is very good that your daughter has felt comfortable sharing her feelings with you and letting you know that she does have a girlfriend at school. It suggests that she feels comfortable with you and the lines of communication are open. So, what to do next?
Depending upon your beliefs and values, you may have different ideas about how to respond to her, but I think that the most supportive response is to let her know that you are glad she shared her feelings with her, you love her no matter what, and you will support her in whatever feelings she has. You can let her know that many people in our society are not comfortable with people who are gay or lesbian and you may sometimes say or do the wrong thing because you know that life is more difficult for homosexuals.
Studies indicate that many kids and teens engage in exploratory sexual behavior with same- and opposite-sex peers. There is a possibility that she may be going through on of these exploratory phases, although there are many, many examples of homosexuals who indicated that they knew at a very young age that they were attracted to same-sex peers. Thus, at this early age, it is difficult to know exactly where your daughter will land with her attraction, but I think that maintaining open and positive communication between the two of you is of great importance.
Finally, I think it is important to consider that ten-years-old is a little young for a child to begin researching sex and naked images. Although many children certainly express romantic interest in peers at this age, I don't see children as interested in the sex piece of romantic relationships as often. Thus, I wonder if she has been exposed to early sexual content through sexually explicit movies, magazines, or conversations with others. It would be worthwhile to ask your daughter what she knows about sex, where she learned about sex, and how much time she spends thinking about it. If she seems preoccupied, you might want to speak with her school counselor to get advice on how to proceed.
I have included a link to an article on talking with your child about sexual orientation below. I have also included a link to PFLAG, which is an organization for LGBT individuals and their families. There are local chapters available where you might be able to get in touch with others who are going, or have gone through, similar situations.