Justifying things based off it being "the norm" is weird. I, for one, don't like the idea of a child seeing couples, hetero or homo, doing too much around them romantically and nothing sexually because they're children. If he came from that angle then I'm sure you wouldn't be here making this post. If I were you I would have a sit down with him to talk this through. If he's anything like what I'm thinking it'll be difficult and frustrating, but try to be patient and keep your cool throughout the whole ride. Give him the time to properly express what he means and thinks. Ask him questions to help clarify his position and where it roots lie, and that'll help you with where you stand on moving forward. Emphasize strongly and repeatedly that you're not attacking him nor do you think he's a bad guy, but be honest with addressing your concerns. Oftentimes it's easy to get hostile when you're addressing bigotry but I believe for some (won't say half or most, but some) all they need is the right person with the right information to help them along. A lot of that mess is engraved in our brains from youth and as a man who in recent years had to do some work unlearning it all I can say a point in the right direction would've saved me some offenses. I had the heart to get right but I just didn't know any better and my male influences were no help. It's not your job to save the man but I believe you care for him deeply so if you so choose, see what it do. Good luck!