I don't think there's enough colonisation or Valoran by other races for interspecies marriage to have come up. I mean if we look at it:
- Ascendants: Limited number and originally humans
- Brackern: Close to extinction; only one member on the surface
- Darkin: Limited number and almost impossible to find seemingly
- Iceborn: Effectively extinct and were originally humans
- Marai: Near mythical species; only one member on the surface
- Minotaurs: Not a large population; on bad terms with humans (generally Noxus)
- Trolls: Limited to the Freljord
- Ursine: Limited to the Freljord
The only other sentient people to even compare to population and influence of humans are the yordles, and even then you have to consider they're limited in locations too; very few Yordles live outside of Bandle City and its surrounding territories. So I don't think the society of another species has progressed for enough for this to actually be a topic anyone in Runeterra's discussed recently.
The only interspecies marriage I can think of is between human and iceborn, which is (afaik) the reason why the latter is effectively extinct. Iceborn were once humans and so they could cross breed, but due to the fact their transformation was magical their children weren't iceborn, but instead humans with some of that magic in their veins. This led to the iceborn's extinction as there was no way for new iceborn to be created following the Frozen Watchers being overthrown.
So to kinda put a cap on this; I'd suspect that due to the limited influence and population of most of Runeterra's native races excluding humans, interspecies marriage isn't actually a thing beyond a few isolated cases. This isn't to say it's not in the cards in the future, especially if yordle society expands in to other parts of the world aside from Piltover and a sole ambassador in Demacia, but right now it's probably something people don't even think about.