Chris:
Though I still differ with you in some of your approach here, I deeply appreciate your careful argumentation. I believe that we are not far apart in this matter.
I have not specifically commented on Ryan's argumentation from the earlier part of I Cor. 7 (I cited v. 5 as possible grounds for Sabbath sexual intermission), but I believe that it is valid, and, in this case, it is at the discretion of the parties. In other words, I do not believe that there is a particular limit on the times when a couple may engage sexually, except when they both agree to do so for prayer and fasting.
Apart from the question of the ordinary propriety of fasting on the Lord's Day (it's not conceived, especially as the day of Resurrection, as one for fasting/mourning but rejoicing), I believe that I Corinthians 7 makes it clear that there is no external circumstance or authority that can impose on a couple a necessity that they abstain from sexual intercourse: it is only their mutual decision to do so for prayer and fasting that creates the circumstance.
Thus, even on the Sabbath, married sexuality can well be construed either as a work of necessity or mercy, depending on circumstances that I could example but prefer not to do here. And I mean by that not hypotheticals, but real and clear instances as to how the expression of love in sex is an ongoing reality of mercy (love, kindness, consideration, concern) and necessity (the I Corinthians 7 text itself makes clear its necessity; again, examples could be given here that would make it clear).
As always, brother, I deeply appreciate the spirit in which you address matters (and you take a back seat to no one in your theological competency!).
Peace,
Alan