Our nephew, who is one of the most respectful and responsible people I know, is marrying his boyfriend.
Do we go out of respect for him, or stay away because of the gay issue?
You could send a card expressing your respect and admiration for him together with your excuses for not going.
My wife has a female cousin who lived together with another woman for 30 years or so. We were never critical of their "marriage", but on the other hand we never showed any hint of approval; all we did was to pray. It's 3 years now that God called our cousin and what a fervent Christian she is! The same may happen to your nephew.