I think it’s (fairly low-level) misogynistic because it’s quite objectifying. Literally, talking about a woman as a passive object that a man inserts his penis into, but even if he’d said “slept with” it’s still likening a woman to an object. It doesn’t directly mention her gender, but it is still gendered because it was said in a social context in which women are frequently objectified and men are not. I’m imagining she found the comment fairly creepy and gross.
I have no issue with mail journalists being the target of jokes, but only in ways that are only offensive to mail journalists and other bad people, not all women.
Personally I don’t find it offensive enough that I would write him off as a hardcore misogynist based on that alone, only if it were part of a pattern (I’m not familiar with his work so have no idea if it is). If someone I know said I’d probably guilty-laugh in the moment but feel a bit uncomfortable and be wary of any other signs that they might have issues with women.