A random person said:
Yes, some people do have "just sex." They're called one night stands.
But no one ever has a one night stand as part of a biological process - if it was for that, they'd do a one night stand to get preggo... but they don't.
It's never just sex. There's always something to it. Whether it's for fun, procreation, or any other reason emotional or physical, humans don't simply do it instinctively.
One could argue that emotions "trick" us into acting instinctively, to which I'd say your point is irrelevant because either way, perceptually we don't do it for frank, biological purposes.