In the end Param never told Hwi that he had sex with Han RIGHT AFTER he agrees for a date....

i thought he did?

That's because he doesn't have to. They weren't a couple at the time.

He had just accepted to go on a date. And he knew Hwi had feelings for him. And they were close / in a sexual relationship.

Had i been Hwi, i would have felt betrayed.

I didnt say he cheated anyway, but even if its not cheating, i think most people would feel betrayed.

You may think of it this way :
You are close friend with a guy
Eventually you confess you feelings and ask him to go on a date
He accepts and the plan is made
But before the date he hook up with someone

This is pretty much what happened except that Hwi and Param were sex friends and Hwi instead.