What about when you want to be with someone and they're in a long-standing relationship, that involves joint ownership of property and so on but you know that it's possible for you to be together but these things are obstacles? Not a situation where someone is cheating or messing about.

How would you manage this so that the third party (for want of a better word, not meaning to be dismissive), say, met someone else and the current partner was ok with that?