You're not stupid. You're just expecting your marriage to always be strong. It's not always going to be strong. When it's weak, women will be there to find chinks in your man's armour.
No, I'm actually not. I know marriages have rise and fall. But even in the fall, I expect my eventual husband not to ever turn to another woman for anything, unless it's his mother. I know this probably gets harder to keep up with time, but it's still a choice on my hypothetical spouses part to avoid any kind of emotional closeness to another woman. I know some women are sharks sniffing out blood, but men aren't helpless creatures unable to avoid advances, and while they may not always make the right choice, I EXPECT them to.
Again, who knows, maybe i'm naive, but I'd rather get blindsided than accept that it's going to happen. That's a sustained misery I couldn't live with, personally.