There are good reasons for chastity:
1) Freedom: A chaste person governs his body as opposed to those simply doing as they wish. The chaste person is not a slave of his sexual desires but knows how to control them. It is important to note that it is not merely about suppression - abstinence - but there is more to it. Chastity allows the person to view other human beings in a different way. The focus is placed on the person, not the physique. Chastity therefore also promotes the dignity of the person.
On the other hand, someone who is not chaste will find himself having to succumb to his primitive instincts. He therefore gives up control over his body and accepts the control of his desires over himself. The unchaste person becomes a slave of his passions.
2) True love: Another good reason for chastity is that it disciplines the person and also filters the desires of a person. In the modernist morally relative society we now live in, sex seems to be a normal part of every relationship - even pre-marital ones. It then gives the person a pure view on the partner: the focus is the person, not the physique - as already mentioned above.
Furthermore, chastity is a great way of finding out whether the foundation of a relationship is true love or temporary physical attraction that may last for a couple weeks. Some couples stay together not because of true love but rather because "the sex is good": such a foundation is not good if one seeks a solid-based relationship. If one then is really seeking Mr. or Ms. Right, then chastity should be an option. If sex is not what the person is seeking, but the person for what he/she is, then chastity should not be a problem: on the contrary, it helps to filter the genuine people from the ones only seeking to satisfy their lustful desires.