Here's the thing...

This movie is about gender roles/assumptions/politics. The rape scene does not particularly bother me in this specific context because it is just one more example of role stereotype expectations being messed with.

1) Guy takes a vow of celibacy = Huge deal! If the character was a woman it would be a non starter as a plot.
2) Guy's celibacy vow for Lent becomes world-wide internet betting sensation = Amusingly embarrassing! A woman's celibacy vow treated the same would result in prosecution for sexual and religious harassment.
3) All of the women are sexually aggressive = Hot! Sexually aggressive men would just be seen as pigs.
4) Guy has issues with his promiscuous sex because he is really in love and wants only to be with his love = amusing premise! For a female character this would be kind of sad.
5) Boss withholds sex from his wife and she's never been happier = ironic humor! A wife withholding sex from her husband would be seen as manipulative.
6) Woman looks a porn sites all day for her job = Turn on! Guy who looks at porn all day would be pathetic (or I suppose some people might think "Dude! You actually get paid to look at porn all day?! Sweet!").
7) Guys bet on vow, post-vow sex, whether guy actually die from prolonged post-vow sex = Wacky things guys do! Women doing the same would just be weird (the only women we see betting for sure are doing so as a power move).
8) Women offering to have sex in order to re-balance the power (and giving the money to charity) = Smoking hot! A guy offering to do the same would be a sleazy creep.
9) Woman takes advantage of her drunk and incapacitated ex-boyfriend and rapes him = Awkward misunderstanding with new girlfriend! Guy takes advantage of his drunk and incapacitated ex-girlfriend and rapes her, and the whole plot turns into a revenge flick about a rapist bastard who is later killed by the ex-girlfriend, her new boyfriend, or both of them together after he somehow manages to avoid prosecution for the rape.

So, if your stance is "Rape is never funny!", then I respect that. But for me, while I don't find it funny, it does not ruin the movie.