Jiggle physics aren't sexist. Adding jiggle physics to increase the sex appeal of female characters is - at least if you don't spend similar effort on male characters. Some might say it's not sexist but rather just business/knowing your target audience, but the latter doesn't exclude the former.

And buying a game knowing it has jiggle physics definitely doesn't mean you're immature. Buying a game because it has jiggle physics might. But that depends on how you define maturity (is it that immature to indulge your sex drive? but that's another topic).