I don't know if the question was a sin, I certainly don't feel comfortable calling it one. That said, it's probably one of those things that you should speak with one's priest about, if you were really curious about it for some reason. He knows you better, and can more accurately gauge what needs to be taught or clarified, what the needs are, etc.
And, as strange as the question sounds to some of us, priests really have heard it all. I know one who was approached by a parishioner regarding her and her husband's difficulty in engaging in marital relations--apparently, they (relative newlyweds) never got any sort of sex ed "back home" and they didn't know what to do with each other, and so he had to teach them some "basic mechanics". They don't teach that in seminary, so I don't know what his research entailed, and I don't want to know.