Disclaimer: I do not own One Piece. Me wishes me did, though.
Zoro was never fond of Sanji. Ever he had met him briefly on the Baratie, he had despised the cook. He knew why, though. It might have been his smooth talking, his way with the ladies, his ability with his legs. But, no, it was neither of these. What he really hated about Sanji was his creativity.
Sure, Sanji could make up a dish faster than you could say, 'Grand Line'. But that wasn't it. He just had a closed mind. Say he was going out with Nami. Not saying he would. Sanji would just sulk in his room for not being able to win Nami over. And there was a problem with his cooking too.
Imagine not using your hands for fighting. That was like suicide for Zoro, because his fighting style was centred on his calloused hands (and his mouth and running in the wrong direction), while Sanji's hands stayed prissy just because he didn't want to fight with him.
It was just pathetic.
So when Zoro went up to Sanji that night and suggested something crude, he was surprised when Sanji agreed.
The two had a fun time that night.