First off, you are NOT a bad person. If you genuinely feel this way, then there's nothing you can do to change it. You can't make yourself look at him differently, and you can't force yourself to feel attraction when you don't.
I honestly don't know what words you can use to tell your husband what you're feeling. There's very little that any of us is as sensitive about as we are our bodies and our attractiveness to our partners. I don't know if there is a way to tell someone they are unattractive that won't hurt. If you are fighting about it, then you are telling him something, and if it's not how you truly feel, then you are lying to him. That never helps. He can't fix a problem if he doesn't know what's broken.
When you do broach the topic of his weight, keep focused on good health. Talk to him about being worried about him, worried about the effects of diabetes or heart disease on his future, and on your kids. 60 lbs in five years is a lot. Has he had a physical? He should probably talk to his doctor about the weight gain, and see if there's anything going on with his thyroid or his blood sugar that would account for the gain.
Let's be honest, going to the gym stinks. Can you work a walk after dinner with the kids into your routine a few nights a week? Is it within your financial means to get a treadmill or an eilliptical for the house? Would he have more interest in playing a sport like racketball or taking martial arts classes for exercise than he is in the gym? Maybe you could trade something off with him for gym time. Like if he goes, you'll give the kids a bath and put them to bed, so that you two can do something together when he gets back. Do you two have a date night? If you do, then aim for more physical activities, like dancing or golf, instead of eating out. What about a challenge to him, like a race. Whoever loses more weight by a certain date gets to pick a weekend getaway wherever they want. Even if you lose, you still win!
Trying to work a healthy eating pattern and more activity into your regular lives will help you out, and will also help him achieve the goal, so that once you do go through with explaining to him what you need him to do, he can succeed. About the only thing worse than being told that your partner doesn't find you attractive would be finding yourself unable to do anything about it.