The "they're a different species from humans, they might not even be able to successfully mate with them" thing really doesn't apply here. These are humanoid cryptid species that are so close to humans that interbreeding is 99.9% on the table unless the author decides to rule it out themselves.
We're getting into "How do you kill Dracula" territory here, but that is absolutely not how species work. Critically, the ability (or, more accurately, inability) to produce viable offspring is a key factor in determining if to animals are of a different species. It's perfectly possible, and even likely, for two similar looking species to be completely incompatible or for them to produce only sterile offspring.
That said, the author could just as easily choose to ignore that fact, as most fantasy series do.
More importantly, it doesn't seem like this new guy is actually the only other titan left, so much as there was a large drop in their population, but a lot of them are still left. As such, this is closer to the whole race traitor, it's your duty to wed a nice -white-titan boy and pump out as many titan babies as you can. Otherwise you're not doing your part and need to be learned a lesson.