Sex is just a by product of marriage, the goal is reproduction.
Once the goal is met, the father's testosterone level will go down, the mother's need is in producing milk to feed the baby. After a few kids are born, both the desires and needs for sexual activities disappear. If the couple wants to cohabitate to bring up children together, they must start living sexless to save resources,energy and time. They have to concentrate on the tasks on hand.
In the pre industrial ages,couples had to continue making children until they couldn't do it anymore. That's why they got to maintain the sex life well into old ages. Now people literally don't need to strive to live that way anymore!
