It's funny that you bring that up because I have a number of gay male friends who are not at all into anal sex or penetration and nobody thinks anything of it. So you're right that it's certainly not the be all and end all of a m/m relationship, but very few slash smut fics don't include it, or if they don't you're still meant to assume it's a part of the couple's sex life generally.

I suspect that this is because for many women penetrative sex is the default, maybe? Which could also account for why there's not more femslash? Maybe a lot of women just have a hard time conceptualizing a relationship without penetrarive sex? I could be way off base there, but it's interesting to consider.