Ok first off I love this story. That being said, it would be more realistic if we found out that they were shapeshifters from alternate dimensions trying to save the world, than it is to believe they've been 'boyfriends' for a year and not had sex. I mean, if one of them confessed to being a vampire it would be easier for me to believe.
I'm not entirely sure what your motivation is for adding this incomprehensible wait time. If this were to happen in real life then they should be awarded the world record for voluntary celibacy between to boyfriends.
I understand wanting to be a closet-case at that age. I was one, big time. But if I were Billy and in this secret relationship we see here, I wouldn't have been able to keep my hands off Brett, much less resist him!
Alas, I will trudge on.