When all else is said and done the purpose of sex is to propagate the species. Without sex there are no people. However, sex becomes an end in and of itself if boundaries are not set. Within marriage, sex becomes a mutual expression of love. Marriage becomes the boundary of a stable relationship that can care and provide for the children that may result from that relationship. Where sex is concerned about the "other" person rather than oneself, particularly in the basic fact that it produces children to be loved and raised in Christ, it is a good thing.
Therefore, the Church, by and large, teaches that children are expected as part of a marriage. This is abundantly clear from the prayers of the ceremony of matrimony. In all these things, sacrifice and moderation are important. The presence of children, by their nature, often limits sexual activity due to the time and attention they require, which provides opportunity for parents to practice self-control. In general terms, sex is expected to become an less important part of the marital relationship, as it matures.
How much is "too much" is difficult to quantify in specific terms, although anything that distracts us from Christ is generally not encouraged by the Church. When sex is for selfish reasons, it becomes a distraction from a spiritual life. If I want to have sex and my spouse does not, I get to practice self-control and sacrifice my own needs for those of my spouse. If she wants physical contact and I'm not necessarily "in the mood", providing for her needs over mine is again a chance to exercise selflessness.
But there are clearly times when sexual activity is proscribed to help us learn and practice self-control and this is clearly outlined by Holy Scripture: