I guess Solas wants elf babies, and ol' Chantry Chump Cullen is just a li'l bit racist.
Well, here's the thing. It might SEEM like Cullen is stepping in poop here with his preference. Initially I was wary of race-gated options too but it doesn't quite apply. At least not in the sense that we have racism in RL (though the concept of it is basically the same)
For one thing, DA-verse has literal races, not skin colors or ethnicities. And in his case, Cullen spent his life with either elves or humans.
Also, you have to keep the racial dynamics in mind. The powerful race is the humans. Had a human been race-gated against a weak and abused race - like elves, then I could maybe see it as racism.
But in this case, he isn't. I think he just doesn't find Dwarves or Kossith attractive. These two latter races are not subservient or slaves, they still have their cultures intact, they're still a force in Thedas. These are not the same social relations as we have today.
Maybe he has more reasons, we'll know once the game is out.
As for Solas, I wouldn't be surprised if he'd want children, might be part of his character, might not. You can't really blame him for wanting his own race or for his sexuality.