Let me try a plausible hypothetical ad...
What if a pro-life church put a similar advertisement with the tag-line "Jesus valued life, and so do we" (with the implication that other churches didn't value life). Would a network be justified to decline this ad?
Would you really defend this principle-- politically charged ads for churches shouldn't be "censored" -- in all circumstances?
I think that an ad like this would be equivalent and that no, a network would not be justified in declining it (assuming that they were not justified in declining the UCC ad, which I guess is my premise).
I feel like the door was opened by all of the political campaign and issue ads that we were barraged with. The UCC ad was no more controversial than any of those. I don't watch enough television to know whether pro-life church ads exist but I have certainly seen billboards with messages like the one you describe, and I don't have a problem with that message being advertised even though I don't agree with it.