I've tried to explain it by asking the person to imagine they have a stomach bug and are throwing up. Would you like to try and make yourself eat some chicken, broccoli and rice while throwing up? Or if they have ever been car sick or sea sick...uh, your not going to go eat a salad and some fish while nauseous. That is the only way i can explain it to the male species.
I just wanted to add that my diet during the first trimester was horrible too. All I wanted was bland, salty carbs. The less nutritional value, the better. I felt guilty about it, but it was out of my control.
It's totally normal - probably something we've evolved to avoid food poisoning, which would have been a real threat for the vast majority of human history. Not much you can do about that!
Just take your pre-natals and eat what you can stomach. You'll probably be able to broaden your diet again once you get past the 1st trimester.