When thanked, are you supposed to say, "You're welcome" and hope the partner leaves a tip??? Like you did the guy a mere favor! I thank people for taking out the trash, not for sex! If a phrase applies to taking out the trash, should it never apply to sex?
Something about the joint venture of sex is relevant to not thanking. Sex is a team sport. Do athletes thank their teammates for playing in the game? Why or why not?