Point to the system that "lets" them adopt ahead of others rather than getting in line with all the rest and maybe we'd agree. They shouldn't be preferred before all others, but they also shouldn't be punished for wanting to care for a child.
The arbitrary point comes when you take a set of "standards" and apply them without consideration for the individual, as in my previous example.
Now, we can find out that Jeff Dahmers are always raised in homes with married white parents and put all of those at the bottom of the list, or you may want to pick some other arbitrary guideline to use....