They'd use the "so you'd only let people planning to have children get married" distraction tactic.
I know no-one can force another person to actually sincerely change their mind. I certainly am too stubborn to let someone try to change my mind too.
We sometimes bang up against this too. My husbands sister, the one he is closest to, is both gay and Wiccan and we all disagree on many things. We are all old enough to have heard each others views many times, so mostly we keep off those topics or just refuse to rise to any deliberate or accidental bait. A few years back she asked if my husband would be celebrant for her wedding, but he said no. He agreed to be photographer, which was a big compromise for him, as 12 years ago he wouldn't attend their Wiccan hand fasting, and didn't really want any part of their wedding. She was not offended this time, and we just smiled through the spells and as he put it 'take pics for her of what makes her happy' which she really appreciated because she knows what that cost him personally. Basically it
takes both parties to be very accepting and forgiving, and committed to having serenity to accept the things you can't change, courage to have hard but respectful conversations, and wisdom to not stuff the relationship up. And pray - I find that makes me less argumentative and less likely to make an idiot of myself. Time is also vital...their views may not change but hopefully they will get off the bandwagon as they work through the issues and be able to relate to you with a different lens