How 2 tell wife 2 lose weight? Tried suggestion, going to gym w/ her, nothing wrks. I'm about 2 have an affair

I love her and want to rekindle our life. She used to be so beautiful. We just had a baby so I know it is hard for her but I am very ually active and need action. There's a girl at work who is giving me signs but I really don't want to go that route. We've only been married for 3 years... I can't imagine 30 more like this. We only have like once every three months. She doesn't turn me on at all anymore. I got us a membership at Gold's. I gained 20 pounds of pure muscle and look very good now. She went for about 2 months and then stopped going. Please help.