Something the kids may not get now (being mostly selfish as all teens are, I certainly was...), the only argument that can be made without appealing necessarily to the Church is this.
Love is defined as always living and doing that which is to the other person's highest good, even if/especially when such an action requires sacrifice on the part of the lover.
A person's highest sexual good is to love their spouse, the one who has publically dedicated their lives to them and who has pledged to be with them forever.
If you are dating someone, and you love them, you will do everything that is to their highest good. Is it the person's highest good to have sex with you or to have sex with their spouse? The only answer that can come from that is B, their spouse, since they are the one who has made the committment. If you love them sexually, to do their highest good when it comes to sex, you will refrain because it is better for that person to sleep with their spouse than it is to sleep with you.