Most slang words for penis (and other slang words describing men) are also normal words. (wood, hard, sausage, pork, balls) This is not the case with slang words for the female anatomy, which usually have no place in daily conversation. Google "slang for penis" and "slang for vagina" if you don't believe me/don't know very many slang words.
Consequently, something a women would say in bed is more likely to crop up in general conversation than something a guy would say in bed.
I really don't see this joke as being sexist at all, and it doesn't objectify women. Who cares if a woman might say that in bed? The fact that the woman is the one talking seems to indicate that she is less of a sexual object, not more of one.
Umm, yeah. It still took one hour of his time, regardless of how much experience he has. If someone rips of this idea, and he loses out, he has lost one hour of his time; not ten years.