In a way this is totally strange as R isn't even two and has no obvious sexual impulses whatsoever. It will be years (probably ten at the minimum) before I need to think about it seriously. But I suppose it's only to be expected, because human society in general has a huge interest (you might even say obsession) with female sexuality: or more properly, the control of female sexuality. Who she dates (or doesn't date), what she wears, what sexual acts she does (or doesn't do), if she uses protection/birth control, her reproductive decisions...everyone has an opinion and is not only willing to share, but many wish to force their opinions upon her (in terms of making various laws, for example: abortion is the most controversial example but there are dozens of others).
R will probably have sex well before marriage, with at least a few others than the one she ends up with (potentially many others), because that's what most people do. While I don't want to know the details of her future sexual explorations (whether with others or solo), I assume that she will have them in spades. That doesn't bother either B or me at all (in which we seem to be very different from most other parents).
I would be terribly bothered by unprotected sex (which is potentially fatal) or unplanned children (which is horribly irresponsible, at least if you are very young and/or single), so I plan to discuss birth control and safe sex early and often. My grandmother took all of her daughters to the doctor at 13 to get them birth control (not because they needed it, but as a preventative), which while rather extreme, is in essence an excellent idea.
My other major concern is about non-consensual sex (sadly a huge problem, especially for girls). R will have to be armed with a strong awareness about sexual predators, ways to avoid potentially dangerous situations (like no drinking/drug use around men you don't know well), self defense techniques, and more broadly, the ability to identify her own desires, and the strength to refuse to do anything that seems unappealing.
Based on my own experience, most of these problems (while of course never the victim's fault) are caused largely by ignorance and low self-esteem. For some reason, most parents of girls feel so much anxiety about their children's sexuality, that they neglect to teach them much (or indeed anything) about it. My own parents, while excellent on the birth control/safe sex front, never informed me or my sister about the existence of lecherous/manipulative men, or how to avoid placing ourselves in sketchy situations (or indeed, what they were). I went out into the world not realizing that going back to a man's house after a date is actually a signal that you are sexually willing (if not necessarily for full-on vaginal intercourse, certainly for something). I don't want R to have to learn all about human sexuality (in all its various forms) the hard way, through trial and error, and while I can't teach her everything (some things must be learned through personal experience), I hope to make her far more savvy than I was.
Knowledge is power, especially when it comes to sex.