Death Over Marriage

Eddie Izzard has a bit where he argues that there could never be an effective Inquisition from the Anglican Church because "cake or death?" would not be a difficult question. But apparently marriage or death is a tricky one for the Catholic Church, or sadly, not that difficult.

The Vatican is opposing a UN resolution calling on countries to decriminalize homosexuality. Of course, in many of these countries this is not only a crime, but a capital offense. So, when we prioritize "thou shalt not kill" and "do not lie down with a man as with a woman," apparently the taking of life just ain't that big of a deal when compared to two men or women living in a caring, lifelong committed relationship. You know, I'm not a member of their club, but the idea that an all-loving Being would agree with them just seems patently bizarre.

I do not want to make a strawman out of this position and really want to understand the strongest possible view on that side, but frankly I am having a hard time constructing it. The argument is:

Archbishop Celestino Migliore said the Vatican opposed the resolution because it would "add new categories of those protected from discrimination" and could lead to reverse discrimination against traditional heterosexual marriage.

How? Exactly how does the legal enlarging of the class of people who may be married affect the class of people who already had the right at all? It is not a scarce resource that gets watered down. If a company sells more stock, then my shares become worth less since they are a small slice of the pie than before the offering, but there is no analogy with marriage here. So, what exactly is the threat to marriage or society that makes ignoring death warranted? I'm not asking for a defence of the position, just the strongest possible formulation here.