Situationships are essentially subscription certificates without specific exercise terms, a romantic relationship wrapped within a friendship framework.

It looks like a call option, feels like a call option, and when exercising, the other party says "We're just friends" (or maybe not even friends).
The essence of a romantic relationship is a perpetual option with a funding rate, where both parties have holding costs, and as long as potential gains outweigh costs, it can be held and exercised multiple times.
Marriage is essentially a lock-up contract with huge exit costs, similar to a fund's LP, large-term deposits, partnership equity, or commercial real estate.
Interestingly, the preconditions for marriage often include large-term deposits and commercial real estate. Choosing marriage too early is like locking all your liquidity, even locking your parents' liquidity, and for most people, even taking out loans to lock liquidity.
This kind of riskless, no-profit investment is not rational, which is why marriage rates are decreasing.
From a more fundamental perspective, why define relationships?
Because defining a relationship is a form of contract. Without a contract, ambiguity prevails, and ambiguity always favors those who are more willing to manipulate the terms.
Relationships will define new rights and obligations, even if non-binding, at least there is a traceable record.
Regardless of the type of contract, are you the buyer or the seller? Are you being exploited by ambiguity, or are you exploiting ambiguity? Are you being exploited by the contract, or are you exploiting the contract?
Many things cannot be explained one by one, but you must understand them clearly in your own mind.