
IMO it's not because they're gay, it's because Yuno has been reacting quite a lot about Kaminsky (we've seen him fanboying a lot about Kami's handsomeness) and Kami really has put A LOT of attention on him (this doesn't mean Yuno liked him 100% sure, nor does justify Kami's advances per se, but I think that's what gave Kaminski the idea that he might have a chance -plus the fact that there's always been this strong aura of attraction between them-).
I do admit that it sounded way too absurd to say "oh, he's gay, let's have s*x now", but I think Kaminsky was kinda trying his luck. Despite how overbearing he might be at times, in the next chapters you'll see more and more that he doesn't want to abuse him (he really wants to protect and take care of him); if Yuno tells him to stop (like for real, he's scared or something) he stops.
I think during the whole story there will be this idea that there is a really strong sense of recognition, attraction, and care in both of them that it just comes out naturally when they're together; and I think is that natural vibe between them, what leads them here (to have s*x and so on). The reason why they did it immediately also has to do because Kaminsky has been dying for Yuno way too hard tho, lol.

i think it's because Kaminsky (K) is coming on HARD to yoon, and while yoon's scared of him and doesn't want him to know he's gay, he's also physically into K. once K figures out yoon's gay, yoon probably thinks "well, what else is there to lose at this point. there's a man who is exactly my type making moves on me... why not."
in my opinion, yoon gets lost in the moment with K (because i mean, if someone who's exactly you're type is lusting after you, you'd probably be pretty on board with it). he even says he's gonna regret it, so he knows this isn't in line with his character. he's just letting his urges take over for the time being.
this isn't to say yoon is "easy" or whatever. he wants his needs fulfilled, and K is there to fulfill them.
Like how did we get there...like ok he’s gay but that doesn’t automatically mean they have to immediately have sex with each other man like we went from gross cousin being annoying and creepy to a full on sex scene between the leads i just...idk maybe i need to re read or smthn to fully understand the pacing...