There was this 90s-2000s TV cartoon show (like the old-style cartoons X-Men, Spider-Man, Superman etc) about an alien being stranded on Earth and fathering a child with a woman. Then something happened to the woman I think (not sure though I don’t remember it); the boy grew up and they were at war with the aliens (I think) and he found out his father was an alien, and it was kinda them-against-the-world things.
It happened in a dystopian world, I think that the first episode happened like the woman enters in a cave after the alien (who looked like human and had the appearance of a human, he had the haircut similar to Gaston from Beauty and the Beast), and the guy says something like "people are chasing me" he probably was hurt, and then she sits with him and they get really close and sleep together. And I think the alien really loved the kid's mom too, he didn't stay in their lives for their own safety and only came for him when the mom was gone.
It was a Western cartoon, not Japanese.
That's all I can remember, because I was really young at the time, maybe 6-7.