I'm not a moral theologian (and if one out there wants to become a contributor, email me, please!), so this is subject to correction. But as I understand it, actions themselves may have objective morality or immorality, and the intentions of the actor may also be moral or immoral. To look at a silly hypothetical, suppose a married couple both suffered from bouts of amnesia. During those bouts they forgot that they were married. If they engage in the marital embrace while truly believing they are not married, have they sinned? Objectively, they are married whether they realize it or not--but in choosing, as an act of the will, to commit the sin of fornication they have in fact, if I am not mistaken, committed that sin.
I'm not a moral theologian either. But hey, this is the Internet, and Red seems to be committing an unintentional equivocation.
As an adjective, "objective" can mean "of or relating to an object as it exists independently of subjective judgment or observation;" this is the meaning it has when it appears inside the adverb in, "Objectively, they are married whether they realize it or not."
But "objective" can also mean "of or relating to the object of a human act," and -- to keep things from being too easy -- the object of a human act is not independent of subjective judgment. As the blessed Pope John Paul II wrote in Veritatis Splendor 78:
The morality of the human act depends primarily and fundamentally on the "object" rationally chosen by the deliberate will... In order to be able to grasp the object of an act which specifies that act morally, it is therefore necessary to place oneself in the perspective of the acting person. The object of the act of willing is in fact a freely chosen kind of behaviour.... [T]hat object [of a given moral act] is the proximate end of a deliberate decision which determines the act of willing on the part of the acting person. [Emphasis, of course, in the original]
In Red's example, then, the object of the amnesiac couple, the freely chosen behavior, is "sex with someone I'm not married to."
As it happens, "sex with someone I'm not married to" is a freely chosen kind of behavior that is always and everywhere immoral independently of subjective judgment or observation. It is "objectively evil" in both senses: evil in its object, and evil independently of subjective judgment.*
What makes the couple's act an act of fornication (despite the fact that, objectively, they are married) is not their intention, but their object. In fact, as given the example says nothing about what their intention might be.
* It seems that "sex with someone I'm not married to" is not a freely chosen kind of behavior as such. Rather, the behavior is "sex" -- which isn't objectively evil in either sense -- and "with someone I'm not married to" is merely a circumstance of a particular instance of the act.
On the contrary, "You shall not commit adultery."
I answer that, both Scripture and Tradition treat adultery and fornication as freely chosen (and of course objectively evil) acts. They are, therefore, specific human acts and not merely the same human act as sex between a married couple under different circumstances. This suffices as a reply to the objection.