Is there an economic value to marital sex that can be "equitably distributed" during a divorce?

Yes. Marital  sex is very valuable stuff. I mean, I don’t mean to be indelicate, but the courts, in determining how much, say, a stay at home mother will receive as a divorce settlement, such things as her home-making, companionship, child rearing duties, Sex, and other variables are considered in determining how much a wife will get in the event of a divorce.
This issue comes up more with executive divorces, where the husband is a powerful CEO type, and the wife was a house-wife who did not contribute as much if at all, to the “economic” value of the marriage. Just because a wife’s contributions are not directly “economic” in nature, does not mean that what she did contribute does not have economic value, or value that can be reduced to dollars and cents; nor does it mean she shouldn’t get a fair share of her husband’s assets – even if he was the sole “breadwinner.” The courts in New York (and possibly other states) will look at her other “contributions” and find a way to quantify and monetize it. Wives of very wealthy men who did not have prenuptial agreements, have been known to walk away with close to 50% of his assets acquired during the marriage, upon dissolution of the marriage – and this could mean, literally hundreds of millions of dollars if not billions.
And one of the big variables is the fact that she contributed, among other things, sex to her husband. Now, analyze that.