Reproduction aside, there’s a lot more to being a man or woman than how your junk works (or whether it works).
When you’re out and about and you meet a new person, you’re not asking yourself “uterus or testicles”? (Nor would anyone have the right or the means to check that, outside of a sexual relationship, thank god.) We don’t have chromosomal test kits that can tell you XY or XX. (Again, for medical privacy reasons, it’s wonderful that our DNA isn’t blasted on displays everywhere we go.)
All you have is what’s right in front of you. “Is this person looking/talking/acting male or female”? And while there are trans people who haven’t yet fully transitioned or aren’t passing well (supplying right-wing transphobic memers with fodder for mockery), there are a *lot* of trans people out there who are passing. A whole lot of them. You’d never know. Once you’ve met a certain number of people, statistical odds are that you’ve encountered at least one and probably several. And you go about your day without factoring these people into your opinions of transgenderism, because you just never notice. You simply reach for the default male/female collection of pronouns based on what your senses are telling you, unless told otherwise.
Last: Are you filling out their government forms? Are you deciding who they’re attracted to and who they date? Do you have access to their entire life memories and experiences? If not, then your ability to contradict another person’s proclaimed gender identity is limited. “You have no standing,” as it’s said. They have a dog in the fight, and you don’t. The answer matters a lot to them and doesn’t really matter to you.