Be Fruitful and Multiply
It seemed wise to start with the very first command that is found in scripture that has a relation to the general topic. Did you know that it is a sin for us to not multiply and fill the earth?
Be fruitful, and multiply and fill the earth, and subdue it. And have dominion over the fish of the sea and over the fowl of the heavens, and all animals that move upon the earth. Genesis 1:28
The above is a command and it is for us. There is no supposed covenantal limitation for this. Let me show you something from the Book of Jasher. It is not inspired, but is twice mentioned in scripture in such a way as to endorse it as far as I am concerned.
Jasher 2:17-22 17 And Lamech, the son of Methusael, became related to Cainan by marriage, and he took his two daughters for his wives, and Adah conceived and bare a son to Lamech, and she called his name Jabal.
18 And she again conceived and bare a son, and called his name Jubal; and Zillah, her sister, was barren in those days and had no offspring.
19 For in those days the sons of men began to trespass against God, and to transgress the commandments which he had commanded to Adam, to be fruitful and multiply in the earth.
20 And some of the sons of men caused their wives to drink a draught that would render them barren, in order that they might retain their figures and whereby their beautiful appearance might not fade.
21 And when the sons of men caused some of their wives to drink, Zillah drank with them.
22 And the child-bearing women appeared abominable in the sight of their husbands as widows, whilst their husbands lived, for to the barren ones only they were attached.
Psalm 127:3 "Lo, children are the inheritance of Yahweh; the fruit of the womb is a reward."
So unlike the teachings of the Gnostics, Ascetics and some of the early church fathers having sex can not be evil or something that should be done as little as possible. If we are to multiply as a people we must have sex with our wives the opposite of what the Gnostics and folks like Tertulian suggested. Before going any further in reading this site I would suggest that you read and do a little study in the book of the Song of Solomon.