For the record, it is not insane for a man not to want to bring another woman into the relationship. I would probably never have a threesome, myself. It's not that there's a jealousy issue, it's that I always want to focus on pleasing my partner. I know it would be damned difficult for me to change focus to another person because I would feel like I was neglecting the one. It is up to the individual. Contrary to popular opinion, it is absolutely not every guy's dream to see their girl with another girl.
In relationships I often am what I consider fiercely monogamous. I give everything of myself to that other person and I will protect that. I get very upset when people do anything to threaten that bond. It's not a possessiveness on my part, I believe it is instead an affront to my partner that they would dare try. Yet I have had wandering eyes before. Many times. I never acted on any of it, but over time, I learned that this was an indicator that something was really wrong in the relationship. I tend to hold people on a pedestal and love them even if they aren't meeting my needs even remotely. I forgive their transgressions to the point that it's detrimental so I refuse to see that a relationship is failing. So when I start desiring the attentions of others... I know there is something very, very wrong. And that's not their problem. It's mine and it's something I work on every day.
I have also been in relationships where quite frankly, we were sexually incompatible. It sucks. I honestly thought up to about a year and a half ago that I had an idealized version of what sex was in my head, that forever reason, I wanted it, but it didn't feel as good as I thought it would. In the relationship which changed that, I never once needed anyone else. While there were things that I wanted that he wasn't capable of to the best of my knowledge, I was happy. Very happy.
But things happened between us, some things that were situational, out of our control. He broke it off because he said he couldn't love me like I deserved. So we went our separate ways to try to find other people. But with every person I saw, nobody was quite like him - I even dated someone that was similar to him in a lot of ways, but it was a cheap imitation. And he went on dates or hung out with a few women, but eventually he realized that for him, there was nobody like me. So he thought he wanted more. We dated other people. And now recently we're back together because he realized what he actually had. He grew; I grew. And while it wasn't an open relationship, we weren't together, that time with other people helped us recognized what we loved about each other.
So no, I'm not directly opposed to an open relationship. I think as long as you trust each other and feel secure that in the end, you will always go home to each other, that you may appreciate each other more. But would I want one? No, but I would rather have an open relationship with trust than lose the relationship all together because I wouldn't bend. And maybe one day, I'd need it, too.