Robert -- As I wrote before, churches are exempt from anti-discrimination laws. They are not exempt if they accept federal funds. So, for example, a Catholic charity that accepts federal funds cannot refuse (where state laws allow) adoption to same-sex parents. In some cases, these agencies have decided either to stop handling adoptions or stop receiving federal funds. There may very well be a handful of churches with white nationalist leanings that would not marry a couple of color and a few more that would not perform an interracial marriage. They have that right and no litigation can force them to do otherwise. But I suspect they are a tiny, tiny minority.
You wrote something assuming that clergy determine whether a church will allow a gay wedding. Not in the UCC. Congregations make the decision whether or not to be Open and Affirming. In most if not all cases, clergy are supportive or, more likely, strong advocates but it is lay people who make the decision. Clergy who don't approve of gay marriage either would not apply to pastor an ONA church or would not be considered for employment at an ONA church.
As I also wrote before, it is this exemption of churches from gay marriage laws that makes one of the strongest cases for such laws -- churches cannot argue that they would be forced to do something they believe is wrong. Instead, equality of marriage is considered a civil issue. Where gay marriage is now allowed in some states, both heterosexual and homosexual couples can be officially married in civil ceremonies and, as a result, have all the legal benefits of marriage. Churches have the freedom to decide, then, whether they will allow religious blessings of those unions in their congregations.
This is why some clergy (in states that do not allow gay marriage) now refuse to perform any official marriages, maintaining (correctly, I believe) that to do so makes them an instrument of discrimination because they cannot provide the same service to all couples, regardless of gender mix. They will bless any union but require heterosexual couples to have a civil ceremony to gain legal marriage rights.