By opening up conversations early and often—as opposed to having “the talk” and being done with it—you’ll communicate to your daughter that you respect her sexuality and the relationships that will go with it, as I gather from your letter you’d like to do. This ongoing dialogue avoids a more shame-based approach (where sex is compartmentalized into a single awkward conversation) and also engenders trust—something you’ll need on both sides as you negotiate boundaries through your daughter’s teen years.
So what will you say? There’s no single “right” way to incorporate our kids’ developing sexual desires into the reality that they’re still young and live in the family household. Every family will have different philosophies and comfort levels around privacy, emotional readiness, and limits. But here’s the point: These should be consistent in a given household, regardless of gender or sexual orientation.
What that means in practice is that there’s no double standard, that your rules don’t change simply because your daughter is attracted to girls instead of boys. Think about what you would do if she were heterosexual. Would you talk to her about sex—not just the mechanics, but safety, peer pressure, readiness, respect, and consent? It sounds like you’ve already done at least some of that. If so, you should have the same conversation with her about sex with women. And if you need to educate yourself about lesbian sex, you might reach out to LGBTQ organizations for resources so that the information you give her is as comprehensive as the information you’d offer her about heterosexual sex.
As for sleepovers, think about what your rules would be if she were attracted to boys. Would you allow boys she was romantically interested in to sleep over? Would you let only boys who were clearly longtime platonic pals sleep over? Would you let a boy sleep over if he slept in the living room? Would you allow a co-ed group sleepover? You might consider what kind of permission your daughter needs in order to have guests over. (“Can Jane sleep over this weekend?” is different from “I invited Jane to sleep over this weekend.”) You can run through this same thought process for any of the parameters you’d have regarding your daughter’s sex life in the heterosexual scenario, such as age for sexual activity, degree of activity, and where it’s allowed in the house (if it is).
Over time, these rules will shift, and the conversations the two of you have as you navigate those changes are how the trust between you will grow. For instance, if your rule is that at age 12 she can have platonic sleepovers only, she’ll need to earn your trust that, say, Stella is really “just a friend” and not someone she has a crush on. The same would be true if this were your rule and she liked boys—you’d have to trust that, say, Simon was “just a friend.” Remember that she will continue to have nonromantic friendships with girls her age, and you don’t want to inadvertently get in the way of those friendships.