It is not just adults that marry. Hetero teenagers can marry so the gays might ask why not gay teens? Then one asks how young a teenager can marry. In many states a 15 year old can marry, but usually, up til now, it happens due to pregnancy. A teen younger than 15 usually needs parental or judicial permission--so could they get permission from their gay parents or a gay judge?
And how about first cousins or siblings? How about an old man and a young boy?
These are stomach turning questions.
I could marry my couch.