There is probably a mix of factors. Its worth noting that the more cosmopolitan the society individuals are operating in, the more acceptable interracial relationships seem to be. People didn't seem even slightly bothered and F'lhammin's husband being an Elezen and we see other pairings like a Lalafel and a Miqote in Limsa.

I think it depends a lot on the culture and social standing. In Hilda's case it was probably more of a big deal because of Ishgard's rather conservative culture and the fact it was a noble involved. Nobles tend to be rather uptight about bloodlines in a lot of cultures.

The fact that there aren't more crossbreeds could be a result of biological issues effecting birth rates and/or just physical attractiveness. While some might be excited by the size and strength of a Roe many would find it highly intimidating. For that matter, you would likely have some pretty serious childbirth issues if any woman of a smaller race got pregnant to a male of a larger race.