That's funny, I was just about to mention something about Sage. On top of being the only girl you can have a second chance with (HOTs party), if you're on Others path, you can comfort her by kissing her after she tells you about her parents' divorce. Makes me wonder, seeing how all the other girls have on-ramps for a second chance as well. Idk it isn't very fleshed out.
Why would they get married in secret?
Well the starting point is Sage getting really anxious about relationships in general. Her parents are getting a divorce, Heather and Tommy broke up, Josy and Maya have problems, Bella's marriage is essentially a failure, etc. So with her graduating in half a year and MC having three more years of college left she doesn't think much of their chances of making it last. MC's first instinct is to reassure her but while he thinks of the best way to cheer her up it hits him that what he is thinking of sounds more and more like a marriage. Before he can shelf the thought Sage picks up on his shift in mood and drags it out of him and they decide that it is what they both want.
But weddings are expensive and since Sage has always avoided taking her parent's money their options are very limited. Building funds is either going to take forever or they have to persuade Sage's parents to pay for it which has zero chance of success. The more they talk about it the obstacles seem to just pile up as Jade and Stephen are bound to object to the whole thing instead of merely not paying for it. So they decide to do it secretly.
They take a look into the legal process and discover that it's actually really cheap and quite fast, if skip on the stuff required for the spectacle known as a wedding. They do need a third party witness though and they choose Cathy. After a lengthy conversation about everything that has happened Cathy agrees to be their witness and it turns out that the most expensive part of the process was treating Cathy to dinner afterwards.
Once it's done they don't really intend to keep it a secret long and indeed tell Neil on the first opportunity. Letting Jade know is a much harder task however. They aren't looking for the perfect moment, but since Jade is having near constant fights with Stephen the chance of catching her in a "good enough" mood is going to take a while. So to avoid Jade finding out from campus rumours they have to keep their friends in the dark. At this point Sage doesn't give a damn about how Stephen finds out.
It then turns out that keeping their friends in the dark is actually very funny and so they start to mess with them. In private they have their serious talks, but in the shared spaces of their houses such as the kitchens they get obnoxiously lovey-dovey. Whenever one of their respective housemates tells them to take their boyfriend/girlfriend to their room they deny that they are dating (which is true since they are married) after a moment of confusion the friend in question accuses Sage and MC of messing with them and gets more confused when they immediately admit that they are messing with them but are nevertheless strictly truthful. One by one their friends eventually catch on but agree to keep the secret when they explain that Jade doesn't know and each of them discovers that it actually is fun to be in on the joke.
The damn thing writes itself.