After the reveal that Mel B. is bisexual, Mel just gets through saying that she wouldn't want to fuck Heidi. In Mel's words "Heidi's too old for me. I prefer them young and perky." She backtracks a little bit to cover her ass, but it's obvious that she's being serious, because she later goes on to again say that she wouldn't fuck Heidi at all.
So after all of that, Howard has the nerve to say that he knows that Mel would want to bang his wife, Beth. He's being totally serious too. Mel, being polite, doesn't outright reject her, but what else could she do? She's not gonna insult the man's wife. Howard takes that as a "yes", of course.
The delusion of Howard to have the balls to somehow suggest that his wife must be the ultimate desirable, after Mel just got done rejecting a supermodel, with the reasoning of her being too old, no less...is amazing to me. Aside from everything else, has Howard deluded himself into thinking that Beth is younger than she is now? Because she's older than Heidi!
It falls right in line with him thinking that Beth is the most desirable woman on the planet. No one can help themselves from lusting after the great Beth-O. Not even his fellow judges, the principal, pilots while they're flying the plane, people who can't stop themselves from jerking off on the subway (even though there are a million pretty women in New York), multiple celebrities, his entire staff, etc., etc.