Since you are a man and not a woman posting, I am assuming you are not thinking of the emotional bond that can accompany intimacy for many woman. I am sure I am not speaking about all circumstances, but generally.
Yes, indeed, I see intimacy as making two people "one flesh." At the risk of sounding old fashioned or overly religious, I believe marriage is essential for all the reasons outlined in the Bible. O.K., I'll admit, just as outlined by the WTS. But of course they are not the only ones to believe such.
I work in a school. If you saw the emotional devastation it causes to children when dad is not around, especially when women have many children from different men because of enjoying "recreational sex," you would feel sick inside. Sometimes I read papers that they have written about how life is not fair for them because dad is not there. I am talking about young children. Second grade. You have seen in the news the actions of older ones due to no father figure. You also know the toll on the economy of supporting children in poverty who do not have a dad.
It causes emotional upheavel in young women who are taken in by young men's "needs" but that do not want a committed relationship. It is hard on girls when they get pregnant because they are the caregivers who then cannot finish school and must make low wages as a single mom. Many times the man walks away with no thought other than the good time he had.
Also, from a medical standpoint, when you have sex with a person you are having sex with every person that one has ever been with. That is why there are so many stds rampant today.
Also, the legal aspects can be bad when children are born or when the relationship turns sour and the woman is left with nothing. People need to take responsibility for their actions.Yes, realize that sex does join you to a person in a special way. Especially when children are involved, and many times of course, getting pregnant is the last thing on the participant's minds, heartache will follow.