It's never been a problem for me. In fact it's a great source of motivation. I love the contrast. For the bulk of our almost 11 years of marriage my wife has been north of 300 pounds and right now is at least twice what I weigh. At her highest she outweighed me by 200 pounds: the difference in our weights was more than I even weighed! And through all that I've never found it hard to stay in shape. I don't doubt your experiences are legitimate, but they're definitely not universal. I'm just one example.From experience I know its really hard to stay in shape when you're dating a fat girl. It's unlikely that a fat girl is going to be down with eating the way needed to stay in top shape. Its also unlikely she'll want to participate in a very active lifestyle.