I've said it before and I'll say it again, - who says the Predator in the second film wasn't female? The bone structure of its head was subtly but noticeably different from all the others we've seen.
But yeah, the boobs look f*cking sucks. It's lazy, makes no sense (most female species on Earth don't have pronounced breasts, why the hell would aliens? Oh, and good luck suckling with dem mandibles!) and more often than not it just leads to stupid, sexualised fan-porn.