Population in the Underground was lowering due to monster infertility.
Then, one day, a human fell, a human with a soul trait completely different to anything the monsters have seen before; lust. The monsters took their soul, and the royal scientist, Gaster, was tasked to study this new soul. He extracted the energy from the soul and was told to inject this into all of the monsters, including himself and his sons (Sansy 'n' Pappy) due to this infertility.
This made everyone have a lust for sexy, sleazy activities. All except Mettaton, Napstablook, the Mad Dummy and the ghost in the dummy in the ruins, as they are either metal (in Mettaton's case) or incorporeal.
Mettaton saw it as his job to re-educate these sex driven slobs via his quiz show, but his ratings were terrible.
Alphys decided to build Mettaton a new body, the sexy body we all know and love.
However, Mettaton is unsure about this new body, and doesn't like it.
But, with some persuasion from Alphys, he decides to use it, and sure enough, his ratings are through the roof for the next show.
But, after this show, he is assaulted by two "fans."
He begs Alphys to change this body, and repair his broken soul, but she can't, so she modifies his body (basically gives him some glasses, as well as minor repairs.)
All he gets next show is phone calls from sluts wanting the sexy dances. He breaks down crying in front of everyone, until, an empathetic Papyrus calls up and answers his questions. Mettaton is overjoyed, and Papyrus tells him that he enjoys the show, and is his "#1 fan" There is a comic dub of this, so I won't explain this any further. Search up "UnderLust, Mettaton's new body"
Now, I don't know if this next part happened before or after the lust injection.
Asriel is born, and Asgore and Toriel are so happy.
Asgore holds the cute, tiny goat baby in his paws.
All of a sudden, Asriel turns to dust.
Asriel immediately dies after childbirth, leaving a distraught Toriel and Asgore to mourn over their son's death.
The dust blows away, to a rose flower, taller than the rest.
The queen walks out into the field of roses on another day and kneels down, to mourn her son.
She looks up and hears a voice.
The tall rose is confused.
"Hello?" It cries out, hopeless.
This is Rosy.
Rosy travels with Frisk on their journey through the underground.
There we are.