Well, I mean, 'gen' does encompass stories which have minor references to sexual relationships - at least, to canonical relationships. I mean, if I write a gen fic about Mulder and Scully's kid, it will probably also reference the fact that hey! They had sex! Because otherwise how would the kid be there? They might even hold hands or something! But that doesn't necessarily mean I'd think of it as MSR, yknow. Actually, pretty much all kidfic is complicated like this, even fic that's about canonical kids. Because it often obligates you to treat the parents' relationship somehow, even if only by signalling that they're happy together in a subtle way, which can make some people feel like the fic's not gen...

Anyway. But of course yes to the "relationships the point of the fic" thing.