There are all sorts of plausible answers:

- Such unions really are infertile, with the sole human/elf exception we've seen in-game. (Nothing weird here - sex without the possibility of conception happens all the time IRL, whether homosexual, post-menopausal, fertility issues, trusting to birth control...)
- The offspring are their mother's race. (I was under the impression the lore said this too; though I dunno where.)
- The offspring are a 50/50 chance of being either parent's race. (This seems to be the case with Garleans / Highlanders; listen to Arenvald and other Ala Mhigan children of Garlean rape being worried about the 3rd eye showing up.)
- They're all in-game, but 'just hiding' (like male Viera, up til now. Plausible since interracial breeding seems rare - we just haven't run into any important ones.)
- We see them all the time, but most aren't identifiable at a glance. (Do note that G'raha must have some Hyur ancestry, unless I'm wrong and ancient Allagan royalty was in fact Miqo'te.)