I'm not a fan of the card, just not funny to me. I don't have an issue with consenting adults engaging in anal sex though.
Interestingly, I saw a card from the same company in my Facebook feed which I thought was awful. It was a birthday card and had a picture of a kebab on the front, with a caption along the lines of 'congratulations on the anniversary of the day you came out of your mum's foof and left it looking like a badly packed kebab'.
To me that's pretty nasty, and distasteful.