I get what you're saying but I never understood why people make a distinction between doing it in your private life vs doing it on the job. Yes, I understand that it's more intimate and personal, as you mentioned. But why is it inherently more acceptable to do when, for example, finding a roommate? I mean a lot of us spend more time with coworkers than family. Is it simply because laws require business practices to not discriminate against protected classes? Or is there something more to this line of thinking? Like a moral justification for the personal case or something like that.

You have a right to control you who live with, that's why.

Sure, but doesn't an employer also have a right to decide to they want to have work for/with them?