Well if it's a venue offering a service, then yes, they may be legally obliged to hire out the venue to evangelical Christians that think homosexuality is sinful on really quite questionable grounds.
Personally I think free speech is so important, the greater good, is that a private company shouldn't always be able to refuse a platform.
For example, should you really have a situation where the Conservative Party can advertise for elections, but all the major companies refuse to take the adverts of the Labour Party? I don't accept that the line, "they're a private company they can do what they like", is always going to be correct.