I sometimes see discourse in English about how romance between adults is supposedly rare in manga. Like, people earnestly explaining that it's hard to make or that Japan isn't interested in it. I realize that various things are biasing what manga they are likely to have access to or hear about, but so much manga about adult romance does in fact exist that I'm confident that anyone who was interested would have no trouble finding plenty of titles worth reading if they went looking for them.
The idea it's rare also seems to go hand in hand with a tendency to put manga about adults on a pedestal. Which kind of raises a suspicion for me that maybe some people want to maintain a self-image of being a person who prefers manga about adults over teenagers, in the face of the reality that they keep reading mostly manga about teenagers, so they'd prefer this to not be a choice in their reading habits, but a consequence of what Japan is publishing?