I've gotten it before as well - just never thought I would get it from my W.
I've gotten hit on by guys in bars and asked that question by a few random people in my past. I had a good friend that I used to hang out with and every now and then people would assume we were a gay couple.
I think part of it is the looks - I am average height, thin, conservatively but well dressed (no backwards baseball caps, etc) with a young-looking face and a short but stylish haircut. My "NiceGuy" tendencies mean I am fairly soft-spoken and polite, don't use rude language or tell rude jokes, etc. I have always preferred to be doing something rather than watching TV, so I never learned to like watching football, etc.
Some people just add it up and say "GAY!"
Me 42, W 39, S8, S6, S2 M 11y, A & ILYBNILWY 11/08 Walking away from a bad situation.