Honestly, I didn't like the story in general. I found it tepid at best. For a situation in which the ML proclaimed "I am going to make you like me/convince you", it's actually the MC that did all of the heavy lifting. He faced the unknown, had to deal with a looming scary version of the future where he looses his closest and dearest friend, came to terms with reality in which he has been nothing but honest for a decade while his friend actually barely told him anything, and most importantly ruminated himself into falling in love. If you really think about it, has the ML actually made an effort and done something tangible? The MC developed his feelings after acknowledging that 1) well, his friend has been in love with him for so long 2) his friend always stuck close to him 3) maybe they really have been too close for comfort and it ruined all his other relationships 4) there's really no other remotely positive outcome for him in this situation 5) he has become somewhat possessive of the ML, most likely because of their lack of boundaries (which, although unhealthy, can happen between friends and family members, people don't necessarily have to be in love for that to occur) 6) the ML's tall and kinda cute. I accept the outcome presented by the author, but let's just say I remain unconvinced. Additional minus points for the refusal to let the MC top despite the MLs awareness of the fact that his friend is used to something completely different (addressed in the first half of the webtoon).
For the oh-so-mature and grown-up gentlepeople in the comments, I am truly happy for all the welcoming and understanding queer communities you seem to refer to and be a part of. Truly. Now, let us examine the actual case at hand and not your anecdotal evidence from your own lives. A person introduces sb to their hopefully-future-partner as a friend and never clarifies the true extent of their relations (lies), that sb also only describes himself as a friend (lies), after which the main character inadvertantly learns that he was the only fool unaware of the situation they were all in. The person that was supposedly interested in him lied to him, kept key information hidden instead of explaining it and getting a clear start. It is nothing but suspicious. If you've ever seen people on the internet ask for help or advice after learning that their partner has been spending time with a "friend" (including one on one meetings) and just so happened to fail to mention that they had slept with each other, the opinion of the overwhelming majority in such situations is that the partner is not a good person. If you keep in touch with your former lovers or fwbs, it doesn't actually make you any better than people who prefer to cut contact. You are just different. We were very clearly shown multiple times that the main character was a romantic that believed he would spend his life with one person, that never had an inclination to have sex-friends, and valued all of the "firsts" and unique experiences he shared with his partner. It is painfully obvious that for such a person it is in fact a big deal if sb they are involved with purposefully withholds such information. It is not immature in the slightest, it is a breach of trust. There are many people like the MC, like me, who put more emotional value on intimate relations because we don't really see them as something outside of some kind of established relationships or something that doesn't mean anything. It is okay if you are different, but if you know your partner in and out and then choose to deceive them about something so important to them instead of disclosing it, you are simply a bad person.