Sooo…not sure why those girls were so intent on Suletta’s intentions with tsun boy when she’s literally the bridegroom to the wealthiest heiress in the universe, like, every single point they made about him applies to Mio as well. Weird interaction there, for a society where same sex couples are supposedly commonplace (from Mio’s perspective anyways).
If we're being charitable, I'd say it's because he actually proposed. And they seem to have taken it to mean there was some prior relationship between the two of them (Chuchu commenting on a lovers' quarrel, and the other girls asking how long they've been involved). Which is a reasonable assumption, at least in comparison to "marriage proposal out of literally nowhere". Also, it's probably common knowledge that Mio-Mio wants nothing to do with the engagement and is trying to break it off.
All of that being said, I understand where you're coming from. Feels like they dropped in the gay engagement as pure bait and have pretty much just left it by the wayside. On one hand it is cool to recognise that same-sex marriage is valid, but on the other hand it was just a plot device and there's not even a modicum of subtext between Suletta and Mio-Mio so far, which is pretty disappointing. I guess it's cool to talk about gay girls getting married but not cool to show it. (I do hope I'm completely wrong, of course. I would love to be proven wrong more than anything!)