I just have this hunch that perhaps Mc is similar to Minwoo's past lover or unrequited love? So he uses Mc to relieve him.
You see when it was during sex min woo is not the usual him like he's hallucinating Mc with some other ppl. I dunno it's just my hunch ╮( ̄▽ ̄)╭
OVERALL I DESPISE MINWOO