It’s oft-repeated but expounded upon. One arena it could be put to the test is with regard to dating preferences. I'm not shy about asking this exact question among my blue tribe friends. There are a few people, almost always bisexual women, who call my bluff and have no hesitation about dating a transwhatever. But that's a niche situation that isn't representative.
While it doesn't matter for most parts of life, if one IS getting naked with a person, then what's in their pants absolutely does matter.