There are male baby showers...it's rare...but it's a thing, and most baby showers are unisex today. The ratings for the show must not be all that great and they needed some publicity.
Regardless of whether "man showers" or whatever is a thing now, the fact is they should have known Rob would most likely have wanted the woman that's actually having the baby there, so it was pretty stupid on their part not to invite her.
