Well technically, the assumption is valid... Heterosexuals greatly outnumber homosexuals. So assuming based on the majority is mathematically sound. It's like assuming that a child will be right-handed.
True, going strictly by population size and distribution, most people are heterosexual right-handers. However, we're not talking about the broad spectrum of humanity here, but a subset that is homophobic. By placing themselves into the category of homophobia, studies have shown that the distribution of heterosexuals among homophobes is less than the general population.
It's like person who declares themselves to be a member or supporter of the KKK. We don't have to know their race to have a pretty good guess they aren't black, Hispanic or Asian. In fact, it'd be a pretty good guess that they are white. Their actions place them in a subset that nullifies known racial distribution percentages for the general population.