Our sexual organs were designed for sex, not our entire bodies. Sex was meant for procreation, the penis fit in the vagina spraying sperm that swims past the cervix in hopes of entering an egg....your anus does not fit anywhere into that equation. Anus = digestive system, the anus fits into your "survival" argument if you want to simply have two human functions
Then explain people who can't get it up, even though they have good blood flow, and biological (physiologically speaking) they have no anatomical abnormalities with their equipment...
Your eyes are designed for sex, you see a female you like, and it triggers your brain to arouse your penis/moisten the vagina (females)...
Our hands help us get off...
We can be aroused by smells, tastes, sounds (why do people get in the "mood" when "let's get it on" by Marvin Gaye starts to play?)
Our entire bodies are designed to enjoy sex...
Your heart beats faster and harder when you are aroused, your brain produces an abundance of hormones that allow to be and stay in sex mode...
If we didn't have all these things in place, there would be a whole lot more men with erectile disorders and women with dry vaginas in this world...
And that would definitely impede our ability to successfully reproduce...