I wonder if it's less to do with seeing homosexuals as inferior, rather the opposition concerns the raising of children.
Marriage, traditionally, was for the bringing up of children, and people worry about giving homosexuals the right to marriage because of their reserves about allowing gay people to adopt.
Personally, I'd say give gay people the right to declare themselves 'married'. It could help people who are nervous about coming out; knowing that their relationship 'prospects' are equal to heterosexual peers will give them the confidence and the pride to reveal who they really are.
Note, too, as long as the two married gay people love the child in question, I think adoption / fostering should certainly be allowed; it's time we move on as a society and practise what we preach about tolerance.