Conjugal fidelity
The married couple forms "the intimate partnership of life and love established by the Creator and governed by his laws; it is rooted in the conjugal covenant, that is, in their irrevocable personal consent." Both give themselves definitively and totally to one another. They are no longer two; from now on they form one flesh. The covenant they freely contracted imposes on the spouses the obligation to preserve it as unique and indissoluble. "What therefore God has joined together, let not man put asunder."
Source: Catechism of the Catholic Church (Vatican.va)