What's with people needing everything explained in detail like it's a hard science/magic manga like FMA? Sometimes things happen. Them having sex OBVIOUSLY had nothing to do with the loop, as evidenced by the slideshow showing many fuckings and suicides. The time loop started without explanation, and that was fine, wasn't it? It could've ended independently from them, or maybe they did do something, but won't know it because they forgot about the loop. The manga has very clearly not been about solving the loop, it's an exploration of how such a thing would affect you. A slice of life in an existentially terrifying situation.
And how can this ending be more happy than it is? They're out of the loop, they don't remember any of the insanity and trauma they went through, yet they still have some residual attraction to each other, which is likely to blossom into friendship and romance. What more could you want in terms of happy endings?