This question is one that Christians are now having to face.
It should not be a difficult one.
Should you go to a celebration, at the heart of which God calls evil? A celebration of something which, in its essential nature, is evil?
Some are already trying to slice-and-dice the question. Every marriage, they point out, will involve sin somewhere. So perhaps we should just abandon the world and live as hermits in caves, to avoid sin?
This is an evasion of the main point. Marriage in itself, in its essential nature, is good, and evils are incidental to its essential nature. Homosexual practice, however, in itself, in its essential nature, is contrary to God's law and to the order of his creation. The evil is inherent and unavoidable in every manifestation of homosexual practice.
There might be some actual marriages a Christian would not attend, because they fundamentally corrupt the nature of marriage itself. If a man abandons his covenant commitment to his wife to take a younger model instead, then we should shun him when he does so. If his marriage is incestuous or bigamous, we should make clear in every way possible that the act of marriage, in this case, is unavoidably and essentially wrong.
If a Christian was invited to a child marriage, in which a dirty old man is to "marry" his ten year-old niece, then presumably the suggestion "perhaps I should go along anyway, to bear witness by my handshakes, smiles and pleasantness to God's grace even to the worst" would be given short shrift in most evangelical churches.
There's no difference to a "gay marriage", except that a "gay marriage" goes even further into depravity - if we let the Bible be our guide as to what depravity is, that is. The only "difficulty" is of societal pressure. We don't live in some traditional African or some Islamic societies where child marriage is accepted and promoted. We feel no pressure to not say "that is an abomination" in those cases.
And that is the only difference. The world has been carrying on a multi-generation re-education campaign to teach us to call good evil, and evil good. We "must" now call truth-telling bigotry, and describe vileness as an affirmation of love, if we are to be accepted by the world. Acting as a Christian in this case rather than in that might cost us something. Being faithful to Jesus might mean taking up the cross and following him. It always has. There is no alternative on offer to his followers.