Why Are Our Genitals Between Our Legs?

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Genitals are between the legs of every vertebrate. In those that don't have legs, it's between where the legs would be (the fins of fish) or where they were (the legs of reptiles that evolved into snakes, which still have them there). They got that way because they're an outgrowth of the excretory tract. In other vertebrates, the excretory and reproductive orifice is the same. It took the mammals to build it into a separate set of genitals.

The excretory organs are at the other end of the trunk from the mouth. That came first; limbs came later. It goes back over a billion years of evolution, to pretty much everything since the jellyfish (including not just vertebrates but insects, worms, and nematodes). Primitive multicellular organisms would eat by absorbing something at one end, processing it, and ejecting the remains from the other. The process of forming the digestive tract through the center of the blastula is one of the first things that happens to a developing embryo.

In a quadruped it's reasonably obvious that the mouth is at one end and waste goes out the other. That's perhaps very slightly less obvious in humans because of their upright posture, but humans are really just very slightly modified quadrupeds. The genitals are there because that's where evolution already had an opening to work with. Evolution rarely introduces new openings into the body, which is why the process of gastrulation (developing the digestive tract) is so fundamental to all complex organisms. It only had to happen once.

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