I always felt that a bigger reason for the (seemingly) large portion of in-shape gays is because we, unlike straights, are physically attracted to our own gender... obviously. In short, besides wanting to be healthy, I know how ****ing sexy nice toned pecs are, so I want them for myself regardless of what my partner(s) look like
Another thing that contributes to it is that out of shape gays probably feel excluded and avoid coming out of the closet for fear they will be paid dust, so it just looks like more gays are in shape when it's really just those ones who felt comfortable enough in their skin to come out and/or be a part of the community.