Those that argue against the Catholic church’s teachings on sexual matters usually only have a few arguments to support their position, which are based around “everyone should have a right to express them self how they want” etc. This is the problem with society; sex is seen as a luxury that everyone should be able to have, which is where the rational arguments for homosexuality come from.
Sexuality is actually a gift from God, whether we use it in marriage or a life of celibacy (such as priesthood). In either case, it requires self-discipline and control. Priests need to stay celibate, so they use their sexuality as a weapon; they deprive themself of (what would seem to be) a basic human need, which allows them to remain holy and close to God and see things such as marriage problems, divorces, mortal sins etc (all of which come to the church often) in a different light. It is a temptation, no doubt, to have sex outside of marriage. This is well known. Our grace comes from fighting that temptation.
It would seem outdated to say that it is beneficial to save ourselves for one special person and give all our sexual love to them. While this is true, most people find it nothing but a corny tagline.
I understand where you’re coming from. Telling people something is wrong just because a religion says so can be open to insult. So, discuss the matter logically with them, with facts such as:
-Marriage is more meaningful, strong and beneficial if it contains an exclusive sexual relationship. Imagine if everybody followed this teaching, you’d be able to say you have a wife that has never been ‘abused’ or ‘stained’ by anyone else, and only you have ever had that privilege with her, nobody else, ever!
-Pre-marital sex ruins your mind. Literally. You get accustomed to taking sex for granted whenever you want, you’ll soon find that your sexuality controls you, not the other way around. This also harms your readiness for ultimately getting married. If you ever would decide to devote yourself to one special person, you’ll find it extremely difficult after having previously had sex with so many different ones. Also, as marriage sometimes requires staying chaste (in times when your partner is ill or pregnant), you’ll find this just as daunting. Imagine that for a second.
-Pre-marital sex is the primary cause for abortions, unloved or uncared for children/orphans, sexually transmitted diseases, divorces etc. Unless somebody wants to argue that one (or all) of these are actually good things, they have no logical case against you for saying that the Church is correct in it’s teachings on the matter.
There, I just rebuked the typical atheistic arguments of modern society on sex without using any direct scripture references. Pre-marital sex is a sin because it is bad for us. The church does not make up rules for the sake of telling people what to do, there is always a rational argument for their teachings.
God bless, and good luck. Pray often for God to give you strength and guidance, and when you find yourself overcome with feelings of temptation and lust, take your mind off it. Think for a second about Jesus’ love for us by dying on the cross, and silently in your mind utter the words “Jesus, Mary, Joseph!”. These words (and quickly meditating upon their meaning; Jesus Chirst, his holy mother the VIRGIN Mary, and st Joseph) are said to grant strength in such instances. God Bless!