Sir.Prize, I think you are fussing over some definition of equality that really carries no meaning whatsoever. Are you and I equal? No, in all sorts of ways -- colour of our hair, eyes, size and shape, interests and so on. Same goes for everyone on the planet. We all are different in often significant ways, and are therefore not equal.
That's really not the point though. Gay marriage is about equality of opportunity, not outright equality per se. The last is impossible but nobody should care anyway, since any two heterosexual marriages are unequal in the same way. The first is massively important, and is what true equality really means. Everyone should have in principle the same opportunities to marriage, education, employment etc. Then the only thing that should change is what they make of those opportunities in practice.
We don't even have this yet, because of inequalities in educational standards, for example -- but I digress. The only reason a gay person should not be able to marry is if they don't have someone who wants to marry them.