As I understand this question, you need to ask questions of the group that is against gay marriage. Presumably, you are asking questions that will be hard for them to answer. Here are some questions that might fall into that category.
- Why is it acceptable to ban a kind of marriage solely because it is repugnant to the religious beliefs of some people? We don’t usually pass laws that are meant only to satisfy some religion’s values. For example, we don’t have laws against using God’s name in vain even though many religions object to that. In that case, why would it be acceptable to ban gay marriage because some religious people object to it?
- How does gay marriage actually harm society? If we are going to ban gay marriage for non-religious reasons, it must be because it harms society in some way. How does it do this?
- If we are going to ban gay marriage because it allegedly weakens the family, why don’t we ban things that truly weaken the family? In other words, why don’t we ban adultery? Why don’t we ban divorce or at least make it very difficult to get one? Why don’t we require premarital counseling to make it more likely that marriages will work out?