This may be a little silly but I was just thinking that since older people are usually a large group do not support same sex marriage and younger people are usually in support of it....4 years of more older people dying and more younger people becoming 18 or older may help some. As well as 4 more years of positive media portrayal of same sex couples in general (Ellen D, popular TV shows with gay couples, more and more people "coming out", more social acceptance) Idk, just a thought.
I was also under the impression that he supported gay marriage but he just couldn't say so / was advised against saying so.
Who knows, maybe they just made Biden into the guinea pig! Lol