Thanks for Doing It

It's our daughter's fourth birthday and my parents take us all out to dinner. We're chatting in the lobby before parting ways, thanking my dad.

Dad: Well, we've gotten a few days of joy out of her in the past four years. Figure we might as well thank y'all. [Pause.] You know, for having sex.
Me: Hell, we'll do it more often if we get free dinner out of it.