He gets less sex than his slimmer self.
Besides, it'd be more enjoyable than a few pumps and having to take a break cos he's out of breath, and also because having a lean, muscular physique makes sex better than realising that your man tits are bigger than the girl's in the bedroom.
Just to clarify, the "If he's getting just as much sex as a slim guy, why bother?" remark was just a joke, a poor one admittedly.
Obviously you should work out for yourself mainly, not just to get some poon. To work out just for women actually strikes me as kind of pathetic. Do it for your health. Sometimes women can provide a nice added incentive though
Slim Guy might also get a higher quality of woman than Fat Guy.