Board Thread:General Discussion/@comment-32129766-20170608022733/@comment-13682699-20170728021630

Asgore would be the more likely candidate for adoptive dad as Frisk has cannonically expressed interest in being his kid when talking to Gerson at the end of the Pacifist Route.

If (big if here) Soriel somehow became a canon pairing, it would not automatically make Sans their adoptive father (IDK about anyone else, but if someone told me to my face that they would kill me without hesitation, I would NOT want them as a father figure.) He would simply be Toriel's lover. Since Toriel was single when she was supposedly adopting them, their adoptive father is technically anyone that Frisk would see/want as their father who also would treat them as their child. In such a case, Asgore would be a far more likely candidate for adoptive dad since Frisk has canonically expressed interest in being his kid (evident when talking to Gerson at the end of the Pacifist Route) as well as Asgore expressing his desire to raise them as his own (though admittedly in a separate timeline he probably remembers it to some degree).