Definitions are weird

I'm like three quarters of the way through an explicit rated fic, having semi skipped several explicit rated activities the pair have engaged in enthusiastically already, and now one of them asks the other
"So when are we going to have sex"

*blinks*

They seem to have quite an active sex life already? That they were enjoying? Thoroughly?
But apparently none of it counts as having sex.

Which happens a lot.
Enough that I bookmarked a soulmates bonding story the other day that didn't require That One Thing for the bond, because duh, finally.

I just really wonder what they're thinking, having a definition of sex that requires a specific configuration of parts arranged in just that one way. What's everything else then?

Makes me want to write explicit just to leave stuff out.