Here is a quote, that happens to be on the subject of premarital sex, but it is a similar situation. This is actually a critique of a position taken by Msgr. Joseph M. Champlin. That isn't you, is it?
Msgr. Joseph M. Champlin wrote an article on unmarried children living together titled "Cohabiting Kids: What's a Parent to Do?" It appeared in the Knights of Columbus magazine Columbia (March 2001), having been reprinted from The Priest, published by Our Sunday Visitor Inc. Champlin warns parents that telling their cohabiting children that they are living in sin and need to go to confession is beyond the pale because it's "playing God."
He does acknowledge that "Living together before marriage is, objectively, a moral wrong. Engaging in sexual intercourse prior to nuptial vows increases the wrongness of that situation." In the next paragraph, however, he opens the door to moral relativism: "But subjective sin represents a different matter. Sin occurs when we fail to follow the divine imperative-in our heart. Sin happens when .... we have failed to follow our conscience." So if, personally, you don't think an "objective" sin is really a sin, then it's not a sin for you.
But this is a gross and irresponsible oversimplification. The Catechism says that "subjective" sin can be truly sinful, and that appealing to "conscience" will not necessarily get you off the hook: "This is the case when a man `takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded by the habit of committing sin' [Gaudium et Spes, 16]. In such cases, the person is culpable for the evil he commits" (#1791).
Sheryl Temaat ---
Still, the act is objectively sinful.
An act that is objectively sinful may not be subjectively sinful, in which case, the person committing the act is not culpable. That's why confessors always take circumstances into account when counseling someone and will ask about the person's commitment to a Christian life overall.