George Weigel. The Truth of Catholicism. Pgs. 96-97

The starting point for grappling with the ethics of sexual love, Wojtyla suggests, is the basic moral truth that we should never use another person for our own purposes, whether those purposes are wealth, ego satisfaction, power, or pleasure.

Loving, in other words, is the opposite of using. 

Everyone recognizes that it is impossible to become a good person by cutting oneself off from others. That is precisely what we do when we reduce others to objects for sexual gratification, even if that gratification is mutual and consensual. We can make love without loving, and we remain essentially, painfully alone when we do. A sexual ethic that calls us to loving rather than using transforms sex from something that just happens into an experience that deepens our human dignity and engages our freedom.  Sex that just happens, even with its transient thrills, is dehumanized sex. Sex that is the expression of the love of two persons- sex that is the meeting of two freedoms in mutual giving and receptivity- is fully human and fully humanizing.

In freely giving myself sexually to another person as an expression of love, I am being myself in the most radical way. I am making a gift of myself to another in a profoundly intimate expression of who I am. That kind of gift, and the receptivity able to accept it, requires permanence and commitment. One-night stands or open-ended relationships cannot achieve mutual self-giving and receptiveness. Without those deeply human characteristics, sexual love is not really love. Attraction by itself does not rise to the level of love. Attraction must be wedded to commitment if the body language of sex is to speak the truth about love. That is the profoundly humanistic reason for the moral truth that sexual love is to be expressed within the bond of marriage.