Even if there is no fear of a STD and the woman ends up not becoming pregnant there is still that period of time where both persons wonder if a pregnancy will occur. Single persons might also argue as they get older that they might never marry and need sex. All people are given free will and have the ability to reason. We are not dogs or cats who instinctively go out and mate. Sexual Intercourse is not a necessity that a person needs to survive.
People can lead happy fulfilling lives without sex.
To say that one needs sex is like saying that we are driven by our desires and cannot control ourselves. This simply is not true. Single people need to address premarital sex very seriously. It is not just a physical act and has many consequences morally, emotionally, and physically. People need to think about their future and not just act upon how they feel at the present moment.