sex is much, much too straightforward for we humans
reproductive organs are unique as far as organs go, in that they contribute nothing to our survival.
they are purpose built for reproduction and do not depend on pleasurable sensations, although they do afford some. there is no actual thing as "sexuality", and there are no "sex organs": no part of the body is designed solely for pleasurable purposes.
you could cite the clitoris, and say its' purpose is only pleasure. and whilst we technically COULD still have sex and reproduce without the clitoris (and its' associated pleasure), we probably wouldn't.
the sad fact is that sex is easy ... but it seems to be much easier if you have a brain the size of a walnut. animals lack the cognitive capacity to sully the reproductive act with a minefield of emotional bullshit. in matters sexual, the paradox of instinct versus conscience is solely a human one.
i'm not suggesting we emulate animals or abandon morality. our history all but proves that the human animal has little taste for self-policing ...
... just that "sexuality" doesn't really exist. it's a convenient umbrella word for all the stuff we do, a lot of it unknowingly, to procreate.
the primal courting riutals of "saying it with the reproductive organs of plants" (and other such "romantic" gestures) are designed to bring about procreation. and the gradual declining thereof is desgined to facilitate its' eventual cessation.
(wow. i'm SUCH a romantic).