A couple can indeed have sex as much as they wish to after marriage. However, as St. Paul says, they can also abstain from sex at times in order to deepen their relationship to each other and to God, and devote their time to prayer as well.
The couple should not use any artificial means to prevent the possibility of conception. The purpose of sex, as mentioned in a previous reply, is both as a unitive act between man and wife and the means to create new life. If a sexual act takes place that purposely destroys and distorts these two aspects it is morally wrong, as it destroys and distorts the natural order that has been created by God.
This doesn’t mean that the only reason for having sex is to create life, but it is one of the major purposes of the act itself. It means that you should be open to creating life, because that is how God has made us. If we aren’t open to life in having sex, there is a major problem with our mindset. The ability to do as God does, and share in His creative ability, is one of the greatest blessing that He has given us. Every newly conceived life is the bringing into existence of a completely new human being: body, mind and soul. The power that God has allowed us in this is completely overwhelming, and to say that we don’t want it is almost to spit in God’s face and say that we know better than He does.
The ability to work with God in the creation of new life should be seen as a blessing, and for that reason a couple should be open to this in every sexual act.
