If we assume that God's intention can be revealed through our "design", then the period of fertility privation that occurs during the menstrual cycle would be a feature and not a bug of the system. In other words, did God intend sex to be infertile during a portion of a woman's menstrual cycle? Because, if he did, the telos of sex during this period is not fecundity because by its very nature the act is sterile by divine design. This is at odds with the Church's teaching. The only way you can square the circle between tradition and our understanding of physiology is to assume that the the infertile period of a woman's menstrual cycle is some sort of privation. But that of course leads to the conclusion that God deliberately produced a faulty product. (There's a whole host of theological problems with that.)
Given the coitus is possible during all stages of the menstrual cycle, what the design of the cycle reveals is that coitus can only achieve its telos of conception during a small portion of it. The rest of the time coitus is intrinsically infertile by design. It would appear that the telos of coitus varies with the stages of the menstrual cycle and the Church's insistence that the coitus is intrinsically orientated towards procreation would appear to be at odds with the findings of physiology.
A sexual act performed during this infertile period is meant to be intrinsically infecund by design. The problem with the idea that sexual activity achieves it telos when conception occurs would mean that woman is intrinsically privated during her infertile period. This would mean that God either deliberately designed a fault (mistake)in women or that he deliberately intended sex to be infertile during this period. i.e. a sexual act performed during the infertile period is teleologically complete and not ordered towards procreation.
Then again, there is the issue of menopause. Did God make a mistake? Is menopause a disease or a deliberate state intended by God? If it is intended by God, then intercourse during this period is teleologically complete and intrinsically not orientated towards children.
As a preface, it should be noted that I’m not Catholic, and not at all well-versed in Catholic theology. But then, I always feel like I can detect a faint whiff of bovine feces every time I get close to Catholic theology. I’m not meaning to disparage the Catholic tradition by any means, but I always get the feeling that the Catholic conception of God is one of a being who is perhaps somewhat more stern and emotionally stunted than the Bible makes him out to be. My own opinion is that Anglican theologians (particularly Lewis and Phillips, though Lloyd-Jones and G. Campbell Morgan are good as well) of the mid-twentieth century had a more correct understanding of God, so I tend to operate more from their theological framework than any other.
That aside, I think there is a glaring, albeit understandable failure in catholic theology regarding sex and procreation. Specifically, the idea that sex is solely for procreation is a rather fallacious assumption. Part of this simply stems from the rather human tendency to categorize things singularly, and part of this stems from a longstanding scientific blind spot.
Succumbing to the tendency to categorize things singularly is somewhat understandable. However, given that most body parts have multiple functions, it seems a little shocking that theologians were unable to conjecture that perhaps even sex itself has multiple functions. For instance, the male penis is designed to deliver semen for the purpose of procreation. It is also designed to channel waste fluids (urine) out of the body. The penis has multiple purposes. The human hand likewise has multiple uses: it can grab, it can be used as a sort of club, it can be used a sensor of sorts, and so on. In like manner, assuming that a certain act is defined solely by its most obvious result is to make the mistake of categorizing singularly. Thus, saying that sex is only, or perhaps just primarily for procreation, is simply a mistake.
The second problem cannot be blamed on theologians. The science of neurology is only now beginning to explain the operation of the human brain, although it still has many major shortfalls. That said, it has recently come to light in the last couple of decades that sex plays a significant role in the pair-bonding mechanism. Specifically, the sexual act triggers the release of a couple of different neurochemicals (if memory serves me correctly, they include but are not limited to: oxytocin, vasopressin, and dopamine), and the effect that these chemicals have on the human brain is similar to that of crack cocaine, which means that people essentially become addicted to their sexual partners. As I’ve noted before, I believe this is what God means when he says that “two become one flesh.”
At any rate, I hope my somewhat shallow and ignorant theological meanderings help to clarify some of the thinking on the matter of the teleology of sex. I think that it’s a little clueless to say that sex is simply for procreation when it clearly is a very pleasurable act (perhaps it might help to have theologians that didn’t view sex in merely theoretical terms, but that’s just me). That doesn’t mean the procreative aspect is irrelevant, but perhaps it would be best to not make it the primary focus. And perhaps it would be best not to overthink things, but that’s a post for later.