Hang on now. There are currently (at least in England) rules about where can be a location to get married. You can get married in a hotel, in a church, in a register office, but you can’t just walk down to the local Burger King and tie the knot. I’m not proposing changing that at all. My suggestion is this: if you are a place which is currently allowed to conduct a marriage ceremony, and you refuse to allow certain types of couples to get married, then you’re no longer allowed to conduct legal marriage ceremonies.
The impression I have is that people who feel that being married in the eyes of the church is the important thing (and oppose gay marriage on those grounds) will be most interested in being married in the eyes of the church, and thus get married in a church; if that church also discriminates against certain types of couples then those two people won’t be legally married, but they don’t care about legal things anyway; they care about the church’s view on marriage. If you care about both, then get married in a church which doesn’t discriminate. If your church does discriminate but you do not and you want to get married in your church, then have a conversation with your priest about why they’re taking the view that they do; maybe you shouldn’t be with that particular church if you disagree on this.
If a couple then says “we want to marry in Place X but Place X won’t let us”, and there’s a Place Y and a Place Z down the road which will let them, then suing Place X is idiotic and they’d get laughed out of court. That’d be like suing McDonald’s for not selling fillet steak.