Species survival is ultimately the basic reason why we human beings search out a mate, or at least that's what the predominant idea has been since our appearance on this planet oh so many years ago. The survival strategies implemented over time have led to the mindset and habits we follow today.
The full range of how normal human sexual activities can be manifested is unknown, although there is more attention being paid to the possibilities than ever before. Survival strategies put into place by religious, political and other controlling factions have had a tendency to put sexual interactions in the class of that which shouldn't be discussed in polite society. Even today attempts to openly discuss sexual issues often elicits nothing more than stammering and red faces.
In order for the human race, or any biological entity for that matter, to survive they must implement survival strategies that ensure they will do so. Species such as ours with minds capable of disseminating information and with the ability to control or allow ourselves to be controlled have created an atmosphere where normalcy revolves around the type of sexual interaction that results in pregnancy.
In order for the human household to function as smoothly as possible there had to be a delineation of tasks and over time the bodily manifestations most suitable to performing those tasks became the desirable sexual characteristics sought after. Put in plain English the women who stayed home, had the children and looked after the home-in whatever form that took-the children, the cooking, cleaning etc. and were capable of producing children relatively easily (those perhaps with wider hips?) became the ones that men sought out. Women on the other hand became cognizant of the fact that they needed a strong, healthy, muscular male capable of going out and either reliably bringing home the bacon (or whatever) or tilling the fields. Protection was another issue and a man exhibiting a frailer physique probably wouldn't do as good a job as one exhibiting a heftier build.
As a result of human survival strategies aimed at the successful perpetration of our species the ideal mate, whether one is seeking a male or female has remained the same for a very long time. Men are 'turned on' by a woman who displays the useful and/or necessary attributes of being able to produce children without too much trouble and take care of the house and be good in bed. Women are 'turned on' by men who are muscular, and powerful ,in other word those exemplifying the characteristics not only necessary in bed but in taking care of the tasks of providing and protecting the household.
In a way it's kind of ironic that entire industries, for example cosmetics, clothing, etc. are based on precepts that came out of what so many sophisticated people would consider base, prehistoric needs. Mode and style of garments are used to emphasis certain physical features, the use of cosmetics to emphasis certain possibly sexual characteristics and make one more appealing to the opposite sex. All these are the result of our species developing survival strategies aimed at making sure that sexual activities predominantly resulted in pregnancy.
Those who might otherwise pursue more selfish sexual motivations that might not otherwise result in the more children and the perpetration of the species felt compelled to instead pursue a course more culturally linked with social acceptance by their peers. Forms of human sexual expression that didn't result in species continuation were relegated to the concept of somehow or other engaging in depraved activities.