May I suggest you contact other people who offer their places for rent through that website, and whom you know to be gay or gay-friendly: let them know of the guy's prejudice against gays (and the opportunity loss for them on that market segment).

They, who are the reason why this guy's business is successful, may send him the message and push him to change his mind a bit. He may own the website, but he is only an intermediary, between people that offer vacation rentals and others that seek them, some of them being gay or gay-friendly.

Not sure if this will help... But the VRBO guy needs to realise he is not the king of the hill and must deal with reality, which gay people are a part of!

As far as your own house goes, there are good alternatives for vacation rentals in France, Italy, Spain or Asia that I know of and surely in other parts of the world, and that are openly gay-friendly. Out traveller may want to mention those in future articles!