Do not lose your temper
The first and most important thing before having an argument with someone is to learn to present your case patiently and not to lose your temper at any cost. No matter what belief you belong to, you must respect the opinion of the other person and cannot just dismiss it out rightly. Do not insult someone because of his or her sexual beliefs.
Do your homework
If you have people in your social circle who are supporters of non-traditional marriages, you should do your homework beforehand because you may get in to an argument at anytime. Try to determine if you actually support traditional marriage due to a solid reason or is it just because you claim to be a traditionalist for the sake of it. Do not indulge yourself in any such argument if you do not have genuine reasons to support yourself. If you really believe in the traditional idea of marriage, you should make points after research to strengthen your beliefs and to convince others.
Most people commit a blunder in conversations by not listening to the other person. This can be one of the major reasons behind a heated conversation as people do not listen to each other patiently.
The purpose of marriage
It is imperative for you to explain the original purpose of marriage in human society. Apart from birth of children, marriage binds a family together and creates harmony in society. You should state that sex is not the only reason behind the concept of marriage. Without traditional marriages, human life cannot proceed further in this world.
If you believe in God, you should back your argument with the help of your religious beliefs and teachings. No religion allows homosexuality and traditional marriages are encouraged.