Monster Marriage is A Complicated Thing
Monsters may be free, but there is a high price to pay for the one that freed them. Deciding Frisk is better off being a mystery, the monsters leave it alone for 14 years. Until Frisk is discovered. Undyne and Sans try to reach her in time, but she is left with a fatal wound. With even boss monsters unable to heal her, Sans heals her the only way left.
However, that was part of Gaster's plan . . . but Flowey was not. The result is wedding bells ringing twice for Frisk. Once to Asriel . . . and once to Sans?
*Frisk and Sans eventually.