Is all fair in love?

I’ve known plenty of people who would not sleep with someone because that person was involved with someone else. Some people seem to follow this code that they would not want it done to them, so they won’t do it to someone else—an instance of the “golden rule.” Or the person refusing to get involved with someone who is already involved could be protecting themselves, since they knew they would only be someone on the side, never having a chance for a long term relationship.

History is full of women “stealing” other women’s men, or men stealing other men’s women. As the saying goes, “all is fair in love and war.”

So whose responsibility is it? If the person with an SO is going after someone else (or open to getting involved with someone else), are they at moral fault, leaving the someone else free to do what they want—perhaps breaking up an otherwise good relationship? Is the fact that the involved person is willing to stray a sign that their relationship is not working? Is it the single person’s responsibility to keep the partnered person on the straight and narrow? What do you think?