This situation has nothing to do with freedoms or rights. Persons A and B become a couple and make a vow to stay faithful to each other, forsaking all others. Pretty standard so far and does not have to involve a formal, legal marriage or include any religious element. Person C comes along and wants to 'mess with' one of them, doesn't matter which one. If one of the persons in the couple is tempted to 'mess' , it involves both persons in the couple. The person who is objecting to the messing is perfectly in their rights to object as an agreement has been breached, an agreement entered into completely voluntarily, not under duress of any kind. If person A gets together with person C, person B cannot be faulted for thinking that both A and C are disrespecting the relationship. Person C cannot be held innocent if they knew about the intimate relationship between A and B beforehand. It is no more egotistical to expect C to respect the couple's agreement to fidelity than it would be egotistical to expect anyone to respect, for example, a family's tradition of always having a Christmas celebration at the old family homestead. The one who thinks this is egotistical is seriously lacking in social mores, is the one who is egotistical.

When someone chooses to interfere with someone else's marriage, then it's no longer a private matter. You mess with my woman, I mess you up. Don't like it? Then don't mess with my woman. Egotistical? Tough shit.