So to set aside playing devil's advocate (that term seems rather apt in context, don't you think?)....
Creating exemptions from laws and regulations is a decidedly tricky thing. Because it does create rather nasty precedents. I imagine if any legislation was put forward it would have to get challenged under common law.
One has to ask the question as to the intention of wanting to be exempt from discrimination laws. Which is why I feel the whole argument is based on hypotheticals. Because honestly, I don't imagine any same-sex couples approaching a parish to get married unless they knew that priest was openly and willingly supportive of it. Same with businesses. One would have to question the sincerity of The Church, given their appalling record of abuses over the years. Like when they try to defend against allegations of abuse..... grrrrr..... 😡
The problem is the humiliation that comes by being turned away.... and I'm not sure stuff like that gets fixed with legislation. Hence, I feel the better way, a way that is more pro-active and prevents the humiliation and trauma, is to create databases of businesses/etc that support SSM, and actively promote and patronise those businesses, so the humiliation is avoided in the first place.