Thank you for the chapter! Here's some critiquing from me:
On page 1, "A men like Seokwoo..." it should be "man" instead of "men". Man is singular (one male). Men is plural (two or more males).
"Dick" is more polite than "cock", but in the middle of sex usually politeness goes out the window, depending on the person's personality. So "cock" should be fine in what's going on now, unless it's a shy timid girl who wouldn't be used to saying it, in which case "dick" would be used.
Also... the very first sentence on the first page "Even when you do it with other women, I'll show you that I'm better than him...!" the 'him' there is confusing to me. She's talking to Seokwoo, the guy, but she's saying "I'm better than him"... so it doesn't quite make sense for it to be phrased that way. If she's talking to Seokwoo, then the 'him' should be 'you'. (Though that doesn't make sense itself...) If she's talking to the other girl, then the first part "Even when you do it with other women" doesn't quite fit... so assuming she IS talking to Seokwoo, you'd want to change the 'him' in this to either 'them' or 'her', since I'm guessing she's trying to say that she'd better than the other women that he would be doing it work. (Also, don't put a period at the end of "Even when you do it with other women."...there should be a comma there instead. Periods are used to indicate the end of a sentence, and how it's written there it's not complete.)
Hope this is helpful!