Is it rude for a man (while in a relationship) to invite another girl over to use his pool? He never talked to his girlfriend about it beforehand?

[Anonymous]

and his girlfriend doesn't know the girl he invited (she has never met her)

yes

no

depends

[Bee-Hatch]

there's not enough to reply give you a well thought out answer here. You've not explained his connection to her. She could be a friend he's known since he was a kid. In which case it'd be OK. Or she could be someone he met whilst out buying groceries, in which case it's not ok. But we don't know

[Plinko]

In general, no.
In specific, maybe.

Not enough context here for a good straight answer. In general though, respect for your significant other dictates you should inform her.