Would it be unreasonable to assume that younger male green riders who may not have completely come to terms with their sexuality might experiment with heterosexual sex on occasion as well?
I think as long as you keep in mind the fact that they ARE gay, and there is no denying the fact that they are gay since they have a green dragon as proof, and no getting around the fact that they are gay, and no changing the fact that they are gay, then its fine. I've had a few male gay friends who were curious at some point, but from what they have told me, no matter how hard they tried or how many times they tried, it just wasn't the same. So I'd say its up to the individual (and this is a Weyr after all, it certainly wouldn't be frowned on), but just keep all that in mind ;)
You might also find that some of them have tried in the past, for example, my male greenrider P'veas at RBW mentions in his history that he experimented with both bioys and girls before Impression. If you actually go look up his posts, you'll see he actually fathered a child.