You have some difficult criteria there. lol
Well, seinen SoL is like
Otomegatari, Emma, Mushishi, Genshiken, Planetes, and Wandering Son.
Onani Master falls into that category but is a bit unique. Survivor's favorite
A Girl by the Sea is also adult SoL. You have fantasy, different time periods, and cultural settings there. And maybe you favor more modern established settings. But damn if those aren't fantastic manga.
Shoujo with natural relationship progress eh... I don't believe
Monkey High had any of those moments but it is light-hearted. This is a tough one because this challenges my memory. Shoujo usually have some sort of "event," usually a possible affair candidate. So that would be stupid reason, drop of the hat. Maybe somebody who has read more of the older shoujo recently can help there.
But ongoing shoujo, I can comment.
Dengeki Daisy is the antagonist to your idea. The only reasons to break the relationship are "real" ones.
Kimi ni Todoke evolves at a snails pace but there are no silly reasons relationships are going south.
Red-haired Snow White and
Reimei no Arcana have strong relationships where only
can affect things. lol
Oresama Teacher doesn't have romance yet but you should still be reading it.