Because then we would also have to count chess players, auto mechanic hobbyists, back woods hikers etc and every other practicer of a chosen behavior.
The constitution does not segregate people by behavior. Only by age and sex.
The main point of the census is to say who lives where, the government probably doesn’t care if they check husband or wife, they’ll just count it as part of “X People live in this area”, along with everyone else.