Exactly, I couldn't find a way to explain so thank you for doing so.
Let's also remember that Leto and Rose's relationship is secret, and maybe that's why their relationship hasn't progressed much and they only showed their affection with kisses, hugs, etc. Besides we can't say that Leto has no sexual desire when he himself at the end of the prologue had a dream with Rose? having sex; and let's not forget that spanking he gave her once.
Only Leto is somewhat naive, and instead of looking for sex, he prefers to wait for things to happen and Rose to be ready; very different from the antagonists who all went for their sexual desires, except Ulrod who did confess his feelings to Cyanna but she rejected him but instead offered her body to him (although we all know she does it to protect Leto).
What I don't remember is if the Duke knows about Leto's relationship with Rose; I think he rather suspects that Rose is fond of Leto and uses that to keep her under control.