What about the word Scion makes it so much more certain that he's descended from Ike? He certainly does have some close connection to Ike, I agree with that, but how does that equal blood descendant?

It directly means a descendant of blood. Just as Owain is to Marth. I.E. Marth's sister didn't have a kid with Merrik and then Lissa and Owain happened. That much is very clear. They set a precedant for the definition it before with Owain.

The reasonas to why I included scion was just in case you wanted to say "the definition of descendant clouds what it really means". So descendant would have questionable and be 50%. The inclusion of scion was meant to say "Hear is a different word that they used. If there was any confusion with "descendant" there shouldn't be any more because the integrity of scion isn't compromised unlike the Anri Marth example did to descendant.

Other than the Mist+Boyd possibility

A descendant of Ike wouldn't be a descendant of those two. That's like saying your great-aunt is your grandmother.

Okay, but let's say this does somehow mean Ike had sex with a woman. How does that make him straight?

It means we have evidence to prove that he is straight. That it means we have more evidence for the "Ike is straight" column than we have for the "ike is gay" column.

He could just as easily be bi. And even gay or asexual people having sex with woman who they aren't sexually attracted to in any way is not unheard of. Surrogate child maybe? Or maybe Ike just had sex with a woman before finding out his sexuality. There's so many alternatives, I just don't think he's straight is in any way an obvious conclusion to draw from Priam's existence.

I actually said this before in the discussion. If that were true, there would need to be evidence. There is evidence for Ike being straight because he has a child. There is no evidence for Ike gay or bi, so that is a mute point. In order for Ike to be asexual, they'd need evidence more than supports. Which can be taken, as I said, either way (the supports).