My husband is 38, almost 39. I tell him that, when he starts to go through his mid-life crisis and is looking for a "hottie," I'm going to be that hottie for him. He can have an affair with me.
Seriously, though, every now and then, he'll pat my rear end and say "Pretty soon, it's going to be all gone and I'll be snuggling up to bones." So I know he worries a bit...but he also wants me to be happy and healthy.
I think that, with so much else in marriage, you have to communicate with each other, offer reassurances, and then communicate some more. Significant weight loss can create so pretty major changes in a relationship...but you have to be determined to weather them with your marriage intact.