Most of us don't, some do, everything is on a spectrum, even gender.

a penis and two testicles are male genitalia regardless

What does the word "male" actually mean? Does it refer to some rigidly definable objective quality that exists in the universe? Or is it a convenient shorthand term that we humans use to describe a set of qualities that tend to cluster together near one pole of what is commonly termed the "gender binary"?

I'll save time and just say it, it's the second one.

So, we categorize human beings (most life forms really...) as "male" and "female". But genitals are not a reliable indicator of whether a person is male or female. Body shape, voice pitch, bone type, hair patterns ... none of those are reliable indicators either. The only reliable indicator of gender is gender identity, aka subconscious sex, aka brain sex. So then what use is it to refer to a particular set of genitals as male or female? If genitals were a reliable way to tell whether a person is male or female, it would make some sense to say that genitals have an intrinsic gender. But they aren't, so it doesn't.

Thought exercise: Imagine a penis just floating in space. Just flesh in a recognizable shape, but entirely devoid of its human context. What does it mean to assign a gender to this piece of flesh?

People say "all penises are male" when they should be saying "most penises are male" or really, "the concept of penis is strongly associated with the concept of maleness". Sure, it's usually convenient to use the shorthand "male", especially when talking about biology. It's easier to say "male" than it is to say "the group of qualities that cluster near this particular end of what we call the gender binary".

That convenient shorthand is what people are referring to when they invoke the word "science" in these arguments. As if it's some kind of proof. Really it's just lazy thinking. Since it's lazy thinking that also reinforces pre-existing deeply held cis-normative views of reality, it's hard for people to shake, and people can get very defensive about it.

But no, not all penises are male.