It's not compulsory, lol.
Entirely beside the point. Yes. You don't have to do it. Still in the game whether you choose to do it or not and still disgusting all the same. Choosing not to do it dies not magically make it go away or shiekd it from criticism on any level.
And while the relationship itself isn't mandatory, the start of the social link, where you use the fact that you accidentally stunmbke into the fact she has a side-gig as a sex worker to blackmail her into being a Confidant and doing shit for you (whether you actually take advantage of that or not, the blackmail itself still happens), is, and that's already kind of super-messed-up, tbh.
Then you have how Joker's/the Phantom Thieves' whole MO is fighting "corrupt adults" but again, even putting the option for a romantic relationship aside, just how the social link starts, if "corrupt adults" are so bad, where does Joker get off starting a "Confidant relationship" under such pretenses to begin with. Especially after Kamoshida, especially after learning what he was doing to people like Ann and Shiho. And then also later all the stuff that happens in the lead-up to Kaneshiro's Palace as well, making it again pretty clear that blackmail is bad and leads to many bad things just compounding in top of more bad things (no duh)... Unless Joker's the one doing it and taking advantage of people, I guess.
Like, there are layers to this one, and none of them are good.