I think it’s interesting how Yuna uses sex as both a carrot and a stick in this relationship. Rena is just horny.
She thinks it's the only thing she has to offer Rena, which is related to her barely-hidden self-worth issues. At least, that's how I remember it, along with Yuna feeling guilty for wanting sex on her own, because then it somehow doesn't "count" as her giving something for Rena's sake. Or something like that.
And that kinda feeds into her irrational worry that Rena is going to cheat on her, because Yuna assumes that if she's not around Rena constantly, Rena will realize how "worthless" Yuna is and leave her. Whereas Rena has been pretty clear, both in what she's said to Yuna and what we've seen of her internal thoughts, that she loves Yuna a lot and already feels very fulfilled in their relationship.
It's definitely getting pretty dramatic with Yuna's slow-motion breakdown and Rena's obliviousness to it (not that I'm blaming her for not noticing, since Rena is actively hiding her own insecurities and doubts), but bad communication and one partner being blissful while the other is freaking out is pretty much a Morinaga signature, so it's not unexpected.