I was curious about why people should even marry. It was not due to me necessarily supporting or opposing marriage, just curiosity. So I looked in the Bible and it is pretty clear about why you should marry,
“But for fear of fornication, let every man have his own wife: and let every woman have her own husband.”
– 1 Corinthians 7:2
“But if they do not contain themselves, let them marry. For it is better to marry than to be burnt.”
– 1 Corinthians 7:9
In these two passages, the Apostle Paul suggests marriage only to those who cannot contain their passions. He states that his life path, that of celibacy, is preferable to that of marriage. The reason being, I presume, that the celibate is called to a life focused upon God where the married person is called to a life focused upon the flesh; their own, that of their spouse, and the creation of a new generation. This is not to say choosing the path of marriage is bad, of course, how else would we “go forth and multiply” without sinning? That a married man is generally not accepted as a (Catholic) Priest is also for this reason; he cannot as intensely focus upon God if he is married.
So then marriage is only recommended for the person who has an intense want to have sex. There are no other clearly stated reasons. But for those who yearn for sex, there is an expectation to rear a new generation. To remain without sexual sin one must also remain abstinent in marriage, which means in no way intentionally preventing the creation of new life. This means no use of artificial contraception or certain acts. For the man or woman unconcerned about living in perpetual sin, they may do as they wish. However, for those who actually believe, it is necessary to abstain from sin.
Men and women are left with two well defined choices. One is a life of perpetual celibacy, free from marriage, fornication, and masturbation, while focusing entirely upon service to God; or a life of marriage with sex, children, and love, while focusing upon the flesh and God. Most people would obviously choose the life of marriage. How many would opt to forgo a husband or wife, children, and sex? Those who would are the select few and they have a particular role to play in this life, one which is just as, if not more, important than those who opted to marry.
If you want to be scriptural, when someone says “sex is not a legitimate reason to marry”, you can tell them it is the only reason we are to marry. Should you not want sex, then you should not marry. It is better to dedicate your life to another vocation which better fits you. Admitting, “Yes, I must marry because I must have sex” is not wrong, it is just honest. If you do not feel that way, then why are you marrying? Why would you marry a person you do not have an intense want to have sex with? Remember that within marriage your body is now owned by your husband or wife as you own theirs and sex is never to be rejected except upon mutual consent to dedicate time towards prayer.
“The wife hath not power of her own body: but the husband. And in like manner the husband also hath not power of his own body: but the wife. Defraud not one another, except, perhaps, by consent, for a time, that you may give yourselves to prayer: and return together again, lest Satan tempt you for your incontinency.”
– 1 Corinthians 7:4, 5