The reason Yuu and Touko's relationship progressed further in the intimacy department is really simple tbh. YagaKimi is just a more realistically written, more grounded romance story than Citrus is. Which isn't necessarily a bad thing, Citrus is what it is — a melodrama. Touko and Yuu had sex because it was the natural progression of their relationship and it just made sense for them to do it. Mei and Yuzu haven't had sex because if they did, it would put an end to the constant will they/won't they sexual tension, which is something Citrus relies on a lot. It's something that MANY "trashy" romance stories rely on. (Trashy romance isn't bad, it's just it's own genre of romance.)
There's a place for both kinds of romance fiction. I won't lie, I definitely prefer YagaKimi I mean... it's my favorite love story ever written. Just saying, there's a reason Touko and Yuu banged while Yuzu and Mei haven't. And the reason is just inherent to the kinds of stories they are.