It really, really is that simple. Don't expect things out of your partner you aren't willing to do yourself. While genetics play a role in your body type, they alone do not produce a tight body. You can be not fat without effort, but no fit person isn't putting in at least a few hours a week at the gym. Get your own ass there or stop expecting it out of others, plain and fucking simple. If I had a donut for every "I take generally good care of myself" I hear out of someone who eats at least 2,500 calories a day and visits the gym 3 times a month, I'd be as fat as they are.

Also, consider this: people who are very fit are very active. They spend a lot of time at the gym, hiking, biking, climbing, playing basketball, swimming - whatever. Their hobbies involve getting up and moving. Yours almost certianly do not (not unless you've got serious overeating issues), or this wouldn't be an issue. How well have your relationships with people who don't share your interests typically gone?