There's a couple of issues here.
First off, I don't think your husband losing weight is going to magically reignite your marriage at all. You say you have no friends and don't make an effort with your own appearance- so I would start there, you sound like you married young and are now very bored, so what about working/starting a hobby/inviting friends over? Make yourself look great, wear nice clothes, do your own hair. This will pay off a lot even if your husband doesn't change.
As for your husband's weight- you married him knowing he was big. He wasn't like me for example, size 8/10 on marriage and at least two three sizes bigger 10 years later. People do get older and most do get fatter as they age.
The thing is, for most people sexual attraction, especially as they mature is far deeper than just good looks, because lets face it, if it was no one would be having sex!
I agree with LEM, if it was all about good looks, most of us would give up having sex in our twenties or thirties! Even if you stay trim, exercise and so on, chances are by 50/60 or beyond you will have wobbly bits and your husband a bit of a gut, even if slim body. I don't know many people this isn't true for, even those who aren't very overweight.
If you had had a great time in the bedroom before you could return to that but it doesn't sound like this was ever the case. I don't think your husband's weight is the issue- I think you are bored and lusting after other blokes tbh.
Even if your husband's weight is the deciding issue, he won't just lose weight. He was big when you met him he's bigger now, this is who he is. If you really can't stand it, you can leave him but I don't think trying to turn an already big guy who was having to do heaps of exercise even when young to keep on top of his weight into a slim one as the years advance is a sensible project. He may well find weight control difficult, my husband does, I guess you have to decide if you love him in spite of this.