Clearly, we need to have kindergartners watching porn or we've let down our liberal overlords.
Seriously though, it shouldn't hurt to teach k-5 graders about relationships. At least to the extent that it's what two people do when they care about each other, and that it can be guy/girl, guy/guy, or girl/girl and there's nothing wrong with that. Basically teach that there's nothing wrong with consensual relationships of any configuration. Middle schools can explain how our bodies work in regards to reproduction, which is how I think it works already (correct me if I'm wrong). High school can deal with sexual health, including birth control & their correct use, STDs, and any other topics I'm not thinking of at the moment.