This is over a month old now, but I'm way behind on a number of things, and it won't get better until the semester is over in a couple weeks. Hugo Schwyzer wrote a while back about a bad policy at what seems to be an emergent-type church involving having married couples promise to have sex every day for a month in an effort to build sexual intimacy. He's probably right in a lot of his criticisms, and I can think of some he doesn't mention. But I disagree with one of his points, and I think it's one he's particularly emphatic about.
He thinks the bad guy here is duty, as if a duty to have sex with one's spouse is bad. His argument is that good things lose their goodness when they become a mere duty. In one sense of what someone might mean by a duty, I think he's right. However, in a pretty common sense of the term, I think he's very wrong, and I think his view is strongly at odds with the moral perspective Paul expresses in I Corinthians 7 and Jesus' teaching in the Sermon on the Mount implies.
It's worth distinguishing between (1) duty in the sense of merely following rules without any further reason and (2) duty in the sense of going the extra mile for another person or doing what you'd want them to do for you if you were the one who strongly desired sexual connection. The first kind of duty is worthy of the Schwyzer's criticism. The second is not.
Suppose one member of a married couple has a strong desire for sexual intimacy, while the other doesn't. I'm not talking about cases of serious illness or complete exhaustion. I simply mean one wants to and the other doesn't. The one who doesn't is completely capable of engaging in sexual activity and enjoying it but simply isn't interested. Now it may be the loving thing to do for the interested party to back down. I don't want to suggest that forcing sex even in marriage is remotely excusable. Nevertheless, the question I'm interested in here is not the moral obligation of the interested party. What Schwyzer was addressing is whether there can be a duty to have sex, not whether there can be a duty to refrain. I'm sure he'd agree with me that there are plenty of instances of that.
The Pauline view is clear on this. In I Corinthians 7, Paul commands husbands and wives to seek to be available to each other sexually except in times of special devotion to intense prayer. That suggests a duty to have sex. It doesn't mean a duty to have sex every night, as the proposal in question suggested. But it does imply a duty to have sex. This Pauline view can be easily motivated by Jesus' teaching in the Sermon on the Mount, particularly by the Golden Rule (do to others what you'd want them to do for you) and the extra mile (if someone asks you to carry something a mile, do it for two miles, and if someone asks for your coat offer up your shirt too). Jesus speaks as if this sort of thing is a typical characteristic of his followers, and those who don't do this are failing to be like citizens of the kingdom of God out to be. I can see how someone would apply such statements to the case at hand by arguing for a duty to have sex even when one isn't interested for the sake of the sex.
But this is not duty for the mere sake of duty. It's duty for the sake of the other person. If a person motivated by love for another person has a duty to do what's loving for the other person, there may well be times when that involves having sex when one otherwise wouldn't have been interested, and Jesus' teaching does seem to include cases like that. I'm not sure why cases of voluntarily being willing to have sex when one isn't interested should be exceptions to the kinds of loving acts he commands in those passages. This doesn't mean setting an arbitrary rule to ensure that couples have sex more often, but it does suggest that the motivation Jesus commands in the Sermon on the Mount involves a duty to show the kind of love that might include things like this. So I would defend Paul against Schwyzer's argument by pointing out that a duty to sex in the Pauline sense seems to follow fairly easily from the kinds of teachings in the Sermon on the Mount that I'm sure Schwyzer has no problem with.