Lasses and lads what is this larping
It feels like soon it'll stop being larping.
Certainly that's the premise. But IRL, I would consider this to be a Very Bad Idea to playact like a couple and family when you aren't one. That's a recipe for misunderstandings about what the other wants. You can have a sham marriage with a good friend to scam the welfare system, but imo you're either a real couple or you're not. Clean emotional lines save future hurt.
This isn't the sole way to interpret emotional relationships. For a counterperspective, A Room For Two posits and describes a happy long term life partnership relationship straddling the platonic and romantic lines, without clear definition. Though unnamed and unlabelled, it is a relationship built on lasting commitment, a shared understanding of each other's feelings, and years of comfortable cohabitation and change.
(There's been endless debate in the fanbase over whether A Room For You is "really" yuri, and if the relationship actually has sufficient emotional commitment from both sides. The point is that the narrative presents itself with the thesis that the relationship is valid, stable, and healthy.)
The relationship presented in "Trying Out Marriage With My Female Friend," on the other hand, is more flagrantly artificial. The last rule of "we are free to break up and pursue other partners" is evidence of a lack of true long-term commitment actively bought-into. Probably the artificiality and conflict in goals is a plot setup for future misunderstandings and necessary clarification of emotional lines.