Title [5/5]

The title is fairly original and made me think 'Whose Game' which is good because it builds interest for the story. 
It also shows the situation of the beginning well. Baekhyun decides to play along to Chanyeol's plan , not quite knowing how things will escalate. Another plus is that Baekhyun actually uses the phrase in an altered form fairly early in the story.

Description/Foreword [7/10]

The description is interesting andy judging by the comments quite well-received. However, it can also be considered a bit limited as it's not very long. It might have made sense for you to elaborate in the foreword, but interacting with your readership (as you did) is also quite important. You can put that kind of thing in an Author's Note next time.
Also you wrote that Baekhyun decides to join while, technically, he didn't have much say in the matter.

Characterization [14.5/20]

Two moderately long chapters don't really leave much space for character developement, so you established what are called static characters.
You characterise Chanyelo as very hormone-driven, bold and determined (or stubborn), while Baekhyun, despite having a more passive role, brings some snark. to the story.
I'm sastisfied that Baekhyun recognizes Chanyeol's non-consensual advances as such, but he seems to forget they are so quite quickly.
For me it was strange that Chanyeol would choose to drop his 'player' ways so quickly and decide to date Baekhyun after suxh a sort time of knowing him. more on that at Plot/Originality.
You didn't put much emphasis on the side characters, which with a two shot is alright.

Plot/Originality [14/20]

I don't think I've ever read a fanfiction about joining the swimm team, so kudos to you !
Despite that , the story wasn't always original.If you read in-between the lines you've got the typical 'jock/playboy who suddenly takes interest in the inconspicuous boy and they develope feelings for each other'.
That actually bothered me quite a bit. They barely know each other(outside of reputation and ), hadn't spoken previously to Beakhyun's joining of the swimm club and shared only meager dialouge afterwards, yet Baek is mysteriously the one to 'cure' Chanyeol of his promiscuity. I understand that this is a type of fan-service but, as it is a M-Rated fic and not really fluff, I honestly think you could've taken some risks.
Concerning the : You write that this is your first , and for that it is remarkably good. You remembered to prep and everything! You did not mention any protection, perhaps to make it more intimate, but it is something I noticed since you emphasized Chanyeol being a player.
Something you mostly neglected in the parts isthat Baekhyun is a in this fic, so it should be pretty painful for him. You could've maybe played on an emotional aspect of this as well.
Something I absolutely loved is that Chanyeol meets Baekhyun's lovely parents. I haven't really read that kind of scene before so it felt quite refreshing.

Mechanics (Grammar, Spelling & Punctuation) [2/5]

You're no native speaker, so don't take this part to heart too much.
Let's start with the spelling .The spelling overall wasn't so bad, but once in a while you forgot some words (ex. 'little he knows').
Your biggest problem here was with words that sound similar but have different meanings (i.e. 'your really cute instead of 'you're really cute' , or 'he's deep voice startled Baekhyun' while the corrected sentence would be 'His deep voice startled Baekhyun').
As for the grammar, you jumped in tenses a lot, especially in your descriptions.
Ex.: 'The dinner was a pretty chaos for Baekhyun. His mom& dad keep telling Chanyeol things that he didn't need to know."
I assume that in the first sentrnce you meant to say 'pretty chaotic' since one can't really measure the attractiveness of. chaos. Chaos also isn't countable.
The revised example is 'The dinner was pretty chaotic for Baekhyun. His mom and dad /kept/ telling Chanyeol things (that) he didn't need to know.'
Sometimes you had some problems with negation (making a word negative). 
Here's an example : 'I didn't you.'
In this case the conjugation indicating past tense is already found in didn't , so all you have to do to write is 'I didn't // you' ( In case you're interested, negation in simple past is generally subject+conjugated negative auxilary verb +infinitive of verb (+object) ).
I didn't notice anything wrong with your punctuation.

Reader's Response [5/5]

You received exclusively positive feedback. Very many readers are requesting a sequel, which means you created characters they enjoy reading about and see potential in.

Overall Enjoyment [7.5/10]

Maybe I come across a bit harsh in this review (in which case I apologise) but I enjoyed this story. It was light reading and I enjoy the dynamics of the Baekyeol pairing. I also like your writing style.
One thing you might want to do next time is add more descriptive scenes.

Bonus [3.5/5]

I loved your three (!!!) posters. 
Subjectively speaking I think your foreword is fantastic.
I liked your hints at KaiSoo. Perhaps their story arc could be featured in a sequel of sorts ?

Total : 58.5/75  (83.5/100)

Comments : Don't forget to credit&upvote the shop!<3