Actually, that's not true. Unlike the title character of Al Franken's book, he doesn't seem to be particularly corpulent. But his "obesity myth" crusade is not just dumb, it is drugged-up-radio-talk-show-host dumb.
The prevalence of obesity among US adults has more than doubled in 25 years. Among US adolescents, it has more than tripled. If a change like that had been observed in prevalence of homosexuality, the scientific consensus would be very different regarding its origins (innate vs. acquired) and probably also regarding whether it is a normal variation or a health problem. But, as an empirical matter, there is no evidence that the prevalence of homosexuality has changed over time. Therefore, reality-based people, who value empirical evidence, do not pretend that obesity is analogous to homosexuality.