The natural purpose of sex is procreation. Procreation requires a male and female since each contain half the DNA that is needed. The pleasure of sex, is like the carrot on the string, that leads the horse to water, so the species can perpetuate.
Pleasure in humans replaced instinctive compulsion, so there would be more choice in terms of a suitable mate, to meet the needs of the primary objective. Once that choice was made desire/love would help to make each monogamous, since children are a long term effort that requires two or more people with different software output; male and female roles.
The problem wth the carrot on the string, leading the primary objective, can be seen with another instinct, called hunger. Hunger is designed to meet the energy and nutrient needs of the body. Like sex, the joy and pleasure of eating leads the body to food so we will eat enough food. Compulsion changing to pleasure allowed humans to become omnivores; choice plus enough variety to meet the primary objective.
The problem is, if you place the pleasure of eating before the needs of the body, many people will eat too much of the wrong tasty things, causing obesity and health issues. You cannot count on intelligence getting it correct, if you favorite food is deep fried lard balls. Intelligence will try to rationalize its pleasure, as this topic suggests. The best foods for the body do not have to be rationalized since they stand on their own based on science. Science can help the primary objective lead your pleasures.
The current fad of fluid sexuality is analogous to using pleasure to justify a diet of fast food, since it contradicts the biology of the primary objective. This is more due a software glitch; psychological, rather than to biological reality.
I trace this change back to social experiments that Liberal parents did on their small children over the past several decades; induced software glitch. The experiments assumed that the traditional sexual roles where a social construct. Therefore, they tried to alter the traditional construct, by having boys play with dolls and other things that had been done with the girls, and vice versa. Now they claim the result is natural. It is a social construct that has induced a software glitch that led to a more dissociated sexual state, than shown by traditional. Traditional was healthier and closer to the primary; real science sex diet.