Still an Analytic: Does Present Duty Reflect Future Loyalty?

I'm an analytic philosopher both by education and by inclination. I love to map out arguments and pick at them until I find their weak spot. Indeed I love the succinct but decisive refutation.

Joe Carter of the Evangelical Outpost says this:

I contend that certain obligations that are recognized after we marry are binding on us even before we meet our future spouses. Although we are separated “relative to temporal perspective” this person exists now and is not in any morally relevant respect different from the person we will wed. The duties of a husband, therefore, would extend not just from the present (when we marry our spouse) and future (throughout our marriage) but also backward into the past (the time prior to our marriage, or even before we meet).

I believe that this admits of a succinct, decisive refutation. Here it is:

Joe assumes that I have a duty of loyalty now to those to whom I will at a later time pledge my loyalty. In particular, he assumes that unmarried adults who will later marry have a duty now to their future mate to have sex with no one else.

This principle (call it Fides) is open to refutation by counter-example.

Let us say that Curtis will marry twice in the future. First he will marry Margaret, and next he will marry Peggy.
Let us assume, moreover, that both marriages are, from the moral point of view, permissible for him. Perhaps Margaret will die and a year later Curtis will marry Peggy.
On Fides, at present and while still single, Curtis has a duty to Margaret to have sex with no one else and he has a duty to Peggy to have sex with no one else.
Moreover, once he marries Margaret (and before she dies), Fides has the consequence that he has a duty to Peggy not to have sex with Margaret.
But this is clearly absurd. The pleasures of the marriage bed are not closed to Curtis and Margaret simply because, at a later time, she will die and Curtis will marry another.

What Joe's principle seems not to take into account is that one can have different loyalties at different times.

Now perhaps Fides can be fixed up. I don't know. But it seems to me more likely that, with all loyalties, the duties entailed by that loyalty do not begin until the loyalty is pledged. Do I have a duty of loyalty to the Marine Corps before I join up? Surely it would be strange to say Yes. (Indeed if the answer were Yes, it seems the Corps could come and haul me away before I signed up. But surely that's absurd.)

This of course is perfectly consistent with the assertion that, before marriage, it might well be wise to remain celibate. (Perhaps if I intend to marry, I have a general duty - a duty not to a particular person - to make of myself a good match; and perhaps sex before marriage will for some reason make me less of a good match.) But the argument seems not to show that.