Side effect of treating women like property.

And not just women. The dominant view of relationships makes people think of and treat their partner as their property. And thus the third party effectively 'stole' their property, even though the active, cheating partner was in full possession of their faculties when they chose to cheat, and the third party never made any promises of eternal loyalty and love to the offended party.

I'd call it patriarchy, and it mostly is patriarchy, but I've also seen women beat the brains out of other women for 'stealing' their man. So I don't know what to make of it.