For me, I wouldn't view attendance as participating in (or endorsing) the union.
Well, when the pastor asks if anyone objects to the union, there could be some awkwardness....
But joking aside, I was livid at Begg for saying this and doubling down. I want to know which grandkid is gay. He preached one of the best sermons I ever heard (the man in the middle).
But as for marriage, I would hold the same for adultery and remarriage. Marriage is also a representation of Christ and His church. I would feel obligated to object and not add myself as a witness to the event. Avoiding endorsing sin at all costs is a generally good principle. This is one of those areas where "being loving" is actually bad and "loving the sinner" is actively encouraging this sin. Does it suck in your family, yes, it does. My aunt has a lovely wife. My grandmother's third husband is someone I love dearly. This doesn't mean that their marriages are valid.