Not sure I’m qualified to answer this, but here are my thoughts:
I have friends who do not share my faith; I have friends that are not religious at all.
I have attended weddings that were not faith based or religious - basically a civil ceremony. Does that make their wedding a sin? Well, maybe to me it would be a sin, but they don’t hold the same beliefs as I do - so to them, of course their wedding isn’t a sin - it’s a celebration of them committing themselves to their relationship (hopefully for a lifetime).
So in regards to a wedding for a same-sex couple, I basically feel the same way - it’s a civil ceremony to celebrate their commitment to one another. Do I feel homosexuality is a sin? I do. But they do not, and they aren’t “celebrating their sin.” Would I bring my children? Probably not, though I imagine it depends on their ages. We can’t shelter our children from everything, and there will be an appropriate time to talk with them about what they’re going to see when they go out in the world (undoubtedly they will encounter same-sex couples, “married” or not).
Now - if I had friends in a same-sex relationship who claimed to be Christian (assuming probably evangelical or one of the Protestant denominations that feels homosexuality is NOT a sin) and they invited me to a church wedding, I would not feel I could attend in good conscience. Yes, in the same way as above, they don’t feel it’s a sin - so really in a way it’s not very different. The difference to me though, is the difference between celebrating a marriage vs celebrating a civil union.
To me, marriage is in the church, with God present, and the legal part of that is the civil union (the marriage license). If you are wed outside of the faith, it’s a civil union (the marriage license) without the true “marriage”. Is that counting on technicalities or definitions? Yeah probably, but really it is the truth. Marriage in the Church is NOT the same as a civil union/ceremony somewhere else, and I don’t expect people who don’t share my beliefs to consider their “marriage” or “union,” or whatever you want to call it, to be invalid or not worth celebrating because of what I believe.
Part of the above reasoning is why I also did not support banning “gay marriage” (or gay civil unions, if you will - because that’s what it is). They don’t share my faith - why should I push a law (based on my faith) on people who don’t share my faith? That is what happens in many countries in this world, and we tend to look down on those countries for being religiously oppressive. I feel I am strong enough in my faith and beliefs that knowing something is legal isn’t going to turn me to “the other side” (so to speak).