Board Thread:General Discussion/@comment-32129766-20170608022733/@comment-13682699-20170728022913

Fastrthnwind wrote: 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).

Sans interactions with Frisk show that his interest in Frisk was mostly for Papyrus's sake as Papyrus's happiness is San's main priority over almost everything else, though a bit was in order to keep his promise as well as making sure that you were doing the right thing (akin to having friend/uncle).