Ok, now I'm starting to see where this is going, but we can't know his age, can we?
The real father was of age X when impregnated Himekawa's mom. Ai was 10 yo when Himekawa was born, she birthed AquaRuby at age 16. That means the real started his relationship with Ai when she was somewhere from 10 to 15 years old.
So we're looking at anything from an early genius playboy who knocked up a woman when he was a teenager himself, then proceeded to Ai who might have been his own age, but then left, and somehow Ai had no ill feelings towards him, instead reacted with lots of understanding.
OR we're looking at someone who was a grown man when he knocked up Himekawa's mom, then went for Ai who might have been barely in puberty and knocked her up when she was 15.
Either way, I now assume that the "not his father" found out about Himekawa not being his real son, then killed his wife and then himself. Considering how dramatic this anime is, it's unclear whether he found out by chance or because the real father sent him a note or something that raised his suspicion.
HOWEVER, was saw in this last episode that the real father is very young looking still. So either he's blessed with amazing genes, or the anime producers suck, OR he might also be involved with some supernatural event in his life (eternal youth?).
In all of this, I'm trying to find a possibility of how he could NOT be a bad guy, because Ai showed now contempt toward the real father, and with how the real father was drawn in the scene we saw, he has that "Kirito-aura", like whatever happened, he's tOtAlLy not at fault. And anime in general shying away from giving us clear-cut villains, especially when they're attractive.
"She's the only non-loli girl in the show, your honor!" will be my defense in court