I have this friend and he says he had no problem with gay people, just the fact that when two guys/girls are together for life, it shouldn't be called marriage. I kinda talked to him about it but I didn't want to get into a big argument so I didn't really dig in.
Maybe he thinks marriage is between a man and a woman. That's kind of about definitions and such, but perhaps more importantly, he isn't super prejudiced against it.
Maybe if they pester you at all, bring up the fact that this same abrahamic definition of marriage he is using also carried the connotations of what was for all intents and purposes gender-based slavery: “Let your women keep silence in the churches: for it is not permitted unto them to speak: but to be under obedience, as also saith the law. And if they will learn any thing, let them ask their husbands at home: for it is a shame to speak in the church.” - 1st Corinthians (14:34-5) “Wives submit yourselves unto your husbands, as unto the Lord. For the husband is the head of the wife, even as Christ is the head of the church.” - Ephesians (5:22-5) “Let the woman learn in silence with all subjection. But I suffer not a woman to teach, nor to usurp authority over the man, but to be in silence.” - 1st Timothy (2:11-5) “For the woman which hath an husband is bound by the law to her husband so long as he liveth.” - 1st Peter (3:1-3) “ye wives, be in subjugation to your own husbands.” - Romans (7:2)