I mean to be fair...

I mean, to be fair, it IS stated that Leo is a dumbass. And he's definitely a narcissist. But you can tell he loved J.J. in his own weird way, but his only solution to keep him around was to offer up 'fuckbuddies' since he thinks J.J. hates him. Is it the best solution? DEFINITELY NOT. But it is his attempt to reach out to J.J. in the "safest" way he can imagine as the dumbass he is. No one ever said he was the sharpest knife (in fact, it seems like the opposite), but he was loyal and he was pretty open about his love to J.J. even if he didn't show it super well. Not to say he's in the right or anything, but from a dumbass's POV, this is the best non-confrontational solution.

As for J.J., he's trying to get Leo to recognize his faults, but he's going about it in a completely wrong way. Clearly their relationship had some issues, especially on Leo's drunk habits, but J.J. didn't say a single thing and then just breaks up with him as 'punishment'. But J.J. did not understand the level of Leo's dumbassery. Some people need to be told straight out what they've done because they don't get that it's wrong or are terrible at self-reflection. Leo, the beautiful idiot, clearly has a healthy dose of both moral ineptitude and narcissistic arrogance leading to an incompetence in basic understanding of the self so absolute he can't even realize he's in love.

J.J. on the other hand, has issues stemming from pent up frustration from actions resulting of said narcissistic arrogance, his own arrogance resulting from feeling special to Leo and thinking he understands him best, and a level 10000 anxiety that culminates in a complete breakdown in communication between not only his lover but humans in general.

One is good at self-reflection but shit at communicating, and the other is shit at self reflection but good at communication. And they BOTH are shit at understanding the other, which is why Leo comes up with stupid solutions like fuckbuddies thinking it's best case scenario, while J.J. gets angry and jealous thinking Leo never gave shit in the first place.