Most people who believe in the sanctity of conventional marriage ...would contend that the homosexuals already have exactly the same rights at present and want separate rights to take their lifestyle into account.

As I pointed out earlier, under US Federal law there are distinct financial benefits associated with being legally "married", and this (marriage) is a distinction reserved (also under Federal law) to heterosexual couples. A heterosexual couple married under what is know as "the common law", which is permitted in a minority of states, can also receive these benefits but must jump through a number of hoops to prove that they are "holding themselves out as a couple" rather than just being shacked up together. A married couple must simply provide a copy of the marriage license as proof.

{For those in the discussion who may not follow the distinction, "Federal" means applying to the United States government's laws and regulations and not to those of the states. State law applies only within that particular state and doesn't overrule Federal laws as they refer to Federal benefits. If a couple is married in a state (eg, Massachusetts) or country (eg, Canada) where such is legal, they are still not eligible for Federal benefits.}

So then, the right that homosexual couples are seeking is the same right that married heterosexual couples currently have: the right to receive benefits to which couples are ordinarily entitled.

BTW, did I see a professed atheist use the term "the sanctity of conventional marriage"? That would be sanctity as in sacred? Sacred to whom/Whom?