I blame Hobby Lobby.
My parents were raised in Christian churches. They got married in my aunt's living room. As best I understand, this was quite a common practice. The elaborate church wedding inspired by British royalty in the 20th century took a while to catch on.*
When I was a practicing pastor, I regularly performed wedding ceremonies for people who wanted a "church wedding," but who were not members of my church, nor particularly Christian. So it wasn't particularly a religious function for them; they just liked the setting.
I've also attended weddings at churches where the participants weren't members, but the sanctuary/chapel/what have you, provided a nice backdrop for the photographs.
Protestants don't even consider marriage a religious ceremony; not officially, anyway. My wedding was in a church, but that wasn't because I thought I wouldn't be married if the preacher didn't officiate in a building designated a "church." (Which is another thing. The Book of Common Prayer has an appendix volume for unusual services, which includes one for blessing, or deconsecrating, a worship space. I know of no parallel to this in any other Protestant tradition. It raises the question of whether the marriage must be conducted in a consecrated space, except really, it doesn't. This whole argument isn't really about religion or marriage; it's about change, and how we have to adapt to it The "holiness" of the service isn't even at issue; it's the fact that the state, which truly controls marriage in America, is making it possible. Yet there is no basis to claim that fact violates one's free exercise of religion.) By the time I married, weddings in a church were the common practice. We did it because that's what you did, not because the space or the pastor made it more acceptable to God.
There is a lot of concern now about "gay marriage," and the ability to object to it. Legally, of course, it's a public accommodation argument: you can't open your business to the public and then refuse to serve certain classes of people. Well, you can refuse to serve LGBT persons, because there is no federal law (and few state laws) that prohibit discrimination against them. But the objection now is settling on marriage, and whether or not you can be "forced" to participate in them.
And "participate" here is part of the skewed reasoning of Hobby Lobby v. Burwell, where providing insurance for one's employees as a benefit of their employment (literally as part of their compensation for their labor) is somehow a burden on the employer if it involves coverage of drugs or treatments the employer claims a moral/religious objection to. Then again, as a society we've never allowed that argument to prevail in cases of discrimination, so even after Loving v. Virginia and now Hobby Lobby v. Burwell, no baker is demanding a right to refuse to cater a mixed race wedding.
Mostly because you wouldn't get very far with that argument in court, or in the court of public opinion. It was tried when the Civil Rights Act was passed. In Newman v. Piggie Park Enterprises, Inc., the defendant claimed: "that the [Civil Rights] Act [of 1964] violates his freedom of religion under the First Amendment 'since his religious beliefs compel him to oppose any integration of the races whatever.' " The court was not too impressed with that argument, and how you could distinguish it now on the basis of a Hobby Lobby v. Burwell claim of freedom from government imposition under the 1st Amendment is a bit beyond me. I suppose you could make the libertarian argument that "the market" would work against such a position of discrimination, but I'm not sure it was "the market" that harassed Memories Pizza into taking a hiatus (or that raised a reported $800,000 in support of its owners. You really have to stretch the idea of "the market" pretty far to hail that as a libertarian success story, on either side.).
This imbroglio will probably soon blow away. If the Supreme Court decides there is no rational basis for disallowing same sex marriages, they will be as protected as mixed race marriages, and at that point I don't think RFRA laws in Indiana or Arkansas will be much protection to bakers and florists, if only because of the social pressure (an amendment to the Civil Rights Act of 1964 is still advisable, because a Supreme Court decision might still not make sexual orientation a protected class). As I say, no one would think today to refuse to cater, say, a Protestant/Catholic wedding, or a Jewish/Gentile wedding. It won't be such a large step to adding that for same sex marriages, in practice if not soon in law.
Eventually this, too, will fade; not as soon as the bad reasoning in Hobby Lobby v. Burwell, but soon enough.
*Of course the British monarch is the head of the church, so it is right and proper members of the royal family are wed in a church. But it underlines the reason Protestants in this country wanted to keep church and state and weddings as separate as possible; because weddings were not essential to Christian practice, and were largely a matter of property rights (for the state) and social order for the community.