Haruka qnd Yuu canonically had sex before Yuzu and Mei.
ST takes line honours but, since Citrus started serialisation around 18 months later even that could be up for grabs in terms of series run time (maybe Citrus just makes it feel longer, lol). It's even more different in-universe: the series started at the start of high school, and at this point Haruka and Yuu have graduated high school (and didn't even actually date, let alone have sex, before graduating) while Yuzu and Mei are still in HS, so in-universe Citrus can (and probably will) beat ST to sex. Haruka & Yuu danced around romance for all three years of HS, and were BFFs in middle school. Yuzu & Mei have only known each other for just over a year.
ST is an interesting case, for most of the series the main pair mechanics are basically S-Class/bait, but the romantic framing (direction, character reactions, etc clearly frame H&Y as a couple) along with Kotone and Shizuku's definitely romantic relationship, allowed it to avoid denial and acknowledge homosexuality without actually doing it. It is also nice that it commented on the uncertainty and resolved it at the end of the main series.
It's one of my feel-good favs, although I wish the anime had been a bit less fanservicey, the excessive focus on bouncing boobs and bare thighs always makes me think twice about recommending what is otherwise a cute, fun show.
