I feel like this explains the Bright and Cheery Amnesiac (and Yamada, from Kase-san, among others), where this weird ambivalence towards sex both drives the entire plot and is held up as some kind of goal.
It's like, For chrissake, just fuck your girlfriend.
This comment doesn't make any sense. For starters Kase and Yamada have had sex, plus the series is still ongoing, so how is that an end goal? Maybe if you're talking about the first part of the series, but I'm talking about the series as a whole.
Yamada's "innocence" is ludicrous. It's not just that she doesn't know the first thing about sex; she acts like she has no idea what it is. She's baffled and surprised every time they go further than holding hands. Kase's physical attraction to her is blatant, but she doesn't seem like she can really wrap her head around it.
The fact that this goes on so long (past the point, as you mention, that they've ACTUALLY HAD SEX) is just bewildering.
And this isn't like insecurity on the part of Yamada; it's this weird ambivalence where the manga portrays a sexual relationship, but one of the participants is simultaneously an innocent ingenue. Yamada barely seems to know they're in an exclusive, sexual relationship... which is kind of the source of the current conflict between them, right? That's why Kase-san is so insecure?
Also with Bright and Cheery Amnesia they literally state early on that they've had sex numerous times in the past, then the amnesia happened and they're rebuilding their relationship back to the spot where they're comfortable having sex again, plus the idea there is that their rolls have somewhat reversed. Which is in my opinion a really fun dynamic between them.
Every other panel of that manga is about how sexy they find each other. It should take one conversation to get Amnesiac to the point where she can have sex; she's portrayed as a super-horndog. But again, the manga presents her inability to grasp the possibility of literal sex as some kind of weird, adorable innocence.
I understand we live in a day and age where many people are convinced that sex is the most in important part of a relationship, if you're not having sex right now something is wrong with you, that's what someone's friends might say or the pop culture media they're consuming with the likes of music and tv.
This doesn't enormously have anything to do with what I'm saying. This isn't about a couple that doesn't happen to prioritize sex; this is about manga writers wanting to show a sexy relationship, but also to idealize some kind of sweet, innocent purity.