I know three boys in their teens and four men who are autistic, from profound autism to various degrees of Alzheimer's expression. I'm sure they all have hormones, but don't have a clue about what to do with them. Even two of the adult men seem to have no interest in romance of any kind.
I'm sure there must be some instances in which autistic children act out sexually, but I'm guessing that's much less common than in the general population. And seriously, I've seen more sexual exploration/play among "normal" children. Some of this is just normal, healthy curiosity, though it certainly warrants parental caution.
If your daughter is okay playing with this boy, I wouldn't hesitate to allow them to be together under supervision, at least until you decide whether you can trust them together. If it were your daughter being excluded because of something she can't help, you'd sure want her to have a chance to socialize and grow as a member of the society we all share. Just a thought.
ADDED: I realize this question is important for me because I'm also on the autism spectrum, although it expresses mildly in my case. But growing up was lonely and difficult for me because I didn't know how to relate to my own age group. I had a wonderful time with younger kids, though, and probably learned most of what I understand about human relationship through play and "mothering" them.