Even if Lilia explained to Takahiro that she has no intentions of raising the next demon king, I don't think he'll believe her that easily, understandably so. She'll have to put in the work to really convince him she means business.
Also, by her logic, 'Someone strong will just be beaten by someone stronger', it could hint that the demons revolve around a 'might makes right' philosophy, meaning for the women of that culture, they require strong, capable men to make babies with, possibly sees such might as an attractive quality. If that's really the case, it would make sense why Lilia has such the hots for Takahiro, who took out the strongest demon in the land, to the point she'd get turned on instantly from the sight of him.
I've heard of a few other stories centered around the romance between a hero and a demon king/royal, but I can already tell this will be interesting. Looking forward to more.