Being oblivious to the way language can be used in subtle ways to reenforce ideas of male superiority and ownership of women doesn't make you sexist, but defending it after it's pointed out sure does.

The sexist aspects of his joke are actually completely unnecessary for the joke you think it is. There's no reason it couldn't be about a friend wondering where she is, or a lesbian coworker.

But the fact that you could take the joke as written, and swap the woman out for a fast car that boyfriend's buddy wants to take for a joyride "before you pick up the money", makes it sexist.

Seriously, break it down: dude propositions woman for sex, woman says no because she belongs to someone else (nothing said about what she wants to do), man insists, offers money, and eventually makes a deal with her owner, man exploits the deal, cheats the owner, and rapes a woman in the process.

But it's funny because it's technically not rape because her owner agreed. And we all know technically-not-rape is the best kind of rape.