I'm confused as to since when Wooju is fine doing that with him aside from the "practicing...

Are you not noticing Wooju is starting to like Jiheon? It's not confusing at all. It's clear asf Wooju is falling for him

I think Wooju always liked Jiheon but not the same way as Jiheon. He treats him almost as if they were in a relationship and forgives him to aesily.
The whole "practice" is odd, i mean he couldn’t have s3x with his exbf but did it with Jiheon many times, then after he was rejected by a girl that resembles Jiheon and the "practice" ends he feels disappointed and keeps having sex with Jiheon.