I would suggest that you try not to worry about this (yeah I know, easier said than done). Unless you think you might have an interest in men, I'd chalk this one up to association- he added the sexual content and your mind automatically thought "feet!" and had a reaction (I suppose especially if you haven't been confronted IRL with actual sexual parts of a woman but now have been with male parts and so you have that visual rather than imagination and that might heighten it?). At least, perhaps accept that as an answer and have some peace about it until such point as it is challenged by further attraction to men.