What doctrine of the Catholic Church covers the commandment, "Thou shall not commit adultery."?

I am having a discussion with my friend about what church doctrine is applied to the 6th Commandment, "Thou shall not commit adultery." I believe it falls under the doctrine of faith/salvation, he believes it falls under the doctrine of marriage. I argue that the last five commandments of "...shall nots" really falls under the umbrella of soteriology and that to single #6 out for marriage would not fit the context of these 5 commandments. Does the Catholic Church have a preference or position on this?