How many gay people are married to someone of the opposite sex?

Too many. But it will be better now that gay men can marry one another.

There really isn't a way to answer this question. The only statistical answers you might find would be either self-reported or, maybe, criminally-reported [if, say, someone who is married gets caught in the same way that was, years ago, George Michael]. Regardless of even the latter condition, it also is a matter of self-definition. Who self-defines as gay, who self-defines as straight, who self-defines as bi-, who self-defines as a-, as trans, as any or many of the abovenoted? How many of these people are heterosexually married? It's not an answerable question. But we do live in a changing world with decent opportunities opening, at least in this way, for most of us in the west.