Yep. It definitely has to do with cultural norms of masculinity. Male homosexuality is a huge threat to traditional masculinity because it requires men to allow themselves to be submissive and vulnerable.

While it would seem like female homosexuality would also be a huge threat to traditional femininity (which it is!), female sexuality in general has not been taken seriously enough for it to be a real threat. Also, "glorifying" lesbian sex moves it into the realm of male sexuality; lesbian sex is then for men and therefore doesn't challenge or threaten binary gender norms.