What they are saying is actually that the argument, the idea that x must be allowed or this group of people won't be able to have kids, implies a selfish motive. Which is rather different than calling particular individuals selfish. People do make that argument, and when it is counter the idea that something is not in kids best interests, it is just saying - what is right for kids is irrelevant if it would affect the ability of these people to have kids.
It's inherently a selfish argument.
There are many people, for example, who think international adoptions are usually detrimental to children in important ways and should be avoided. You can disagree and say, no, they aren't, and maybe have some reasons to support that POV - not a selfish argument. If you say, on the other hand, these things you are saying about the child's best interests aren't the point, the point is that it would mean childless well-off western couples can't adopt, or gay men don't have as much chance to adopt - that is a selfish argument. It is about the good of the parents rather than that of the kids.