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?
And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth 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.
Psalm 127:3 "Lo, children are an heritage of Yahweh: and the fruit of the womb is his 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.