Do you feel like you have to watch your SO to make sure they aren't cheating?

I just asked a question about why a woman wouldn’t want to be involved with a bisexual man. A number of people mentioned that they’d have to not only watch out for other women, but they’d have to watch out for other men, as well.

One commenter wrote, I do think I wouldn’t be able to marry a man that’s bisexual, though. I’d have a difficult time being able to tell if he’s just hanging out with his buddies or if he misses being with a man that he’s “hanging out with his buddies.”

There were others who said things along similar lines—that they’d have twice the work trying to make sure they remained faithful. This struck me as odd. Since when is it your job to keep your SO faithful? Isn’t it their job to do that? Well, obviously, some people think they have to keep a sharp eye on their SO, or he or she will do something that sets them on the road to cheating. It’s as if the SO is property, and they have to make sure the property isn’t stolen.

So I ask you—do you feel this way? Do you know anyone who feels this way? If you feel that way, why is it your job to keep an eye on your partner? What do you think about trust? Why can’t you trust your partner to stay faithful?

I guess this goes along with the idea that your partner shouldn’t have a friend of the opposite sex. That’s asking for trouble. Don’t trust it. Now there’s no one he or she couldn’t have a relationship with. So don’t let him or her have any friends at all?

Where does this kind of thinking come from? Can someone explain it to me, please?