Back before same-sex marriage was legal in the United States, I would always ask straight people who opposed it:

"How will me being allowed to marry someone of the same sex [as me] remove your right to marry someone of the opposite sex?"

However, at that point in history, I at least had some respect for people who supported the concept of universal same-sex civil unions even if they were uncomfortable with the word "marriage."

Now that it's the law of the land and we can see it poses no harm to society, retaining our right to same-sex marriages is nonnegotiable.