Nerdy: grab dice and char. sheets (yay!)
Sexy/Nerdier: Skill check, have in character sex!
Sometimes it turns out hot and surprisingly natural. Sometimes it's disastrous.
As for non-nerdy based bedroom RPing, I'm not sure I've ever done it. I don't think I can really stomach "okay you're a cop and I'm trying to get out of a speeding ticket" without a well-written character back-story #_#. I might be too nerdy to live.
My boyfriend and I love role playing. I'll usually pick a costume/character to surprise him with and he just follows into the scene. We're both actors, so it's not really difficult or out of the ordinary for us.
(As far as being nerdy goes, our favorite characters to get freaky with are Batman and Harley Quinn. Nerd life!)