After watching the Karlach/Dragonborn date scene my first thought was that there should be no romance for the lizards, unless with other lizards.
But why though? As a spontaneous thought some might think that way, but that's no reason to disable something like this? Larian never shyed away from more exotic combinations if you have a look at DOS2, and I am glad they let their players decide how they want to play. As others already said, the romances are purely chosen by the player and a player's Dragonborn can like what they like. Aside from that, I'm absolutely certain that in a world like the Forgotten Realms there would be enough people who are open for these kinds of relationships. "There's enough furries and monsterfuckers who would go for a lizard. " as Silver/ so aptly put it. You can see for yourself with a brief google-search if you don't believe us, viewer discretion advised
And if humans are like that then Dragonborn would be too.
Btw, looking at the live chat during the date scene, there were PLENTY of people eyeing the Dragonborn, he'd have a free choice of partner.