God has a plan for His children which involves helping us live happy lives and developing God-like attributes. The plan started with Adam and Eve in the Garden of Eden:
Genesis 1:27-28
27 So God created man in his own image, in the image of God created he him; male and female created he them.
28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth,
The commandment given to Adam and Eve to multiply and replenish the Earth applies to us today as well. A couple that does not consist of a man and a woman cannot fulfill this commandment.
Genesis 2:24
Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
Here God gives us a commandment which goes along with the one about multiplying and replenishing the earth. Why would God give us these commandments if they weren't for our own well being?
Marriage between a man and a woman constitute the basic building blocks of society. It is in this framework that children are most likely to get the love and care that they need to be healthy individuals.
A healthy society is made up of healthy families.
One of the concepts in ethics is universalizability. Which is to ask what if everyone did it? If we all turned to homosexuality, you can see that in a hundred years, the earth would be de-populated. Thus, God's purposes would be thwarted.
Family is important to God. Look at the promises made to Abraham in Genesis 22:
17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies;
18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.
Our culture values living for yourself without regard for what is good for society. With no fault divorce, if either partner in the marriage decides its too inconvenient to stay married, then a divorce is granted. This concept of marriage for convenience extends to incorporate the concept of same sex marriage. In both cases, the idea is to grant as much liberty to adults without regard to how it affects society at large, or children specifically.
The God of Christianity is a God of sacrifice. He sent his only begotten son to redeem his children. Just as Abraham was commanded to sacrifice Isaac, God wants us to give him a broken heart and contrite spirit. How can we learn to do that when our natural tendency is to live for ourselves? By entering into marriage and having children, we learn to sacrifice and gain a perspective on life that is tough to gain in any other way.