What's the deal with Rush Limbaugh's anal rape references?

You kind of have to wonder why Rush Limbaugh keeps making references to anal sex as a metaphor when talking about Barack Obama. First it was this:

Then yesterday it was this:

Ben Armbruster noted at ThinkProgress back in January that this is a pattern for Limbaugh:

Limbaugh on support from African-Americans and gays for progressives: “Democrats will bend over, grab the ankles, and say, ‘Have your way with me.’”

– Limbaugh on Alaska Governor Sarah Palin’s (R) so-called “Troopergate” scandal: “This is pure sexism in Alaska on the part of these old boys trying to get rid of Sarah Palin, and she didn’t put up with it, and she didn’t bend over and let them have their way.”

I just think it's a wee tad peculiar when a guy like Limbaugh with a known history of sex tourism, always seems to go to the well of anal penetration when talking about Barack Obama.