There's nothing in Zen Buddhist teaching that prohibits lay people from having responsible sexual relations.
Sex is like any other area of human activity. It creates an opportunity for us to watch our minds and see how desire, anger and ignorance control our behaviors.
Because sexual intercourse can bring up deeply hidden feelings, it provides a context for discovering equally hidden intentions. When we become familiar with the intentions behind our actions, then we can make choices around them. The ability to choose consciously liberates us from suffering.
That's the inner meaning behind the Third Precept (to avoid actions undertaken in lust). Even when lust appears, we can learn from it - if we choose to study the feeling and reveal the intentions behind it.