In order for this issue to actually hurt people's ability to live in an area, it would have to be a majority of landlords doing this, not just one or two.

I think in certain regions, that actually may be the case. If we're averaging over the entire US, probably not; but in certain counties or states, yeah, I do believe it could make a difference. And even if it's rare for an area to have zero landlords willing to take a same-sex couple as renters, having like half or even a quarter or 10% of landlords in an area being like that would still lead to that couple expending more energy than an opposite-sex couple just to find housing.

While I know personal incredulity is not a real argument, that just seems unlikely to me.

Honestly, it would be ideal to have some data on the extent of the issue but I haven't done enough reading about this area to know. If I get a chance, I'll look into it.