Same-sex marriage could be viewed as nonsensical

But this argument is irrational. If two things are inherently different they cannot be equal, even if the State or a majority decides so. For example I love running and I envy Usain Bolt. But I’m never going to be equal to him, even if the State were to bring in a law declaring me his equal. The State would merely be party to a nonsense.
Indeed many individuals identifying as homosexual recognise the inherent male/female nature of marriage and have expressed satisfaction with the civil partnership option in this context.