Every demographic has some disposable income. And yes, sexual minorities tend to be childless, and heterosexuals tend to have children. So all other things being equal, sexual minorities tend to have a higher percentage of their income available for discretionary spending. The same can be said of DINKs, and single heteros.
There may be other factors. Obviously gay people tend to be employed in some professions more commonly than others, if for no other reason than the majority members of those professions are less homophobic. Are those professions better paying? I don't know for sure (I can think of anecdotal examples either way), but there's a possibility, in which case it's possible that gay spending power isn't entirely reducible to the presence or absence of children.
Another factor might be this: While there's no reason to think that poor people are any less likely to be gay, it DOES seem possible that poor, uneducated gay people might be less likely to come out than better-educated (and thus, more affluent) gay people. So by definition, gay people targeted by advertising campaigns (i.e., openly gay people) would be wealthier.
Or think of it this way. People in the J.Crew target market have some spending money, and some of them are gay, and the ones who aren't mostly don't care. Some Walmart shoppers are gay, too, but appealing to them in ads is more likely to ruffle feathers among the non-gay Walmart customer base.
But yes, children are very expensive, so not having any would boost one's discretionary spending ability.
