Hello, how did you're "Frans" fall in love with each other ? Because if there was a génocide before, what did Frisk so spécial to make sans in love with them that much ?
Frisk didn't do the genocide run quickly, instead, they went through several neutral routes, confused, lost, and not sure how to survive in this unfamiliar landscape without hurting others. As they slowly started to understand this place, and themself, they were able to reach their pacifist run.
Answering why Sans fell in love with them back then is easier, especially since many Frans stories follow this idea. Sans has been depressed for years, thought nothing would ever get better, but despite this makes friends with an old lady who he, not wanting to break her heart, promises to protect the next human that falls.
Despite the human feeling threatening (he doesn't remember resets- but as I've said, neutral runs, gets phantom memories), they turn out to be really nice, improve the lives of his friends, and give him hope for the future again.
That, and the human has the power of resets. He can't tell people about the timeline stuff he knows about because he feels like it's better not to burden other people with the knowledge he has, but with them he can discuss it freely.
Plus, it's a little flattering to have a demigod be into you.
Papyrus actually sets them up in this timeline. It's cute! They've got a very fluffy relationship...
Which gets ruined when Frisk does a true reset with the goal of killing everyone.
Sans can't even feel betrayed, because he doesn't remember the other timelines well enough to.
"Frisk is the only person who truly can understand what Sans has gone through, because they cause it and because they remember it"
Also I think intense fighting is pretty gay :P