If it sounds like they have quite a few reasons against supporting marriage equality
It sounds like this might be a difficult question for you. Do you want to talk to me more about that? What makes it difficult?
If this is difficult for them, they might need time to talk through their questions. If they’re not wrestling with it as a question then they have probably made up their minds.
Don’t take this personally. It can be hard when people you know don’t see things in the same way or share your values. But all it means is that for now they are someone who cannot take that last step with you on this issue.