Sexual intercourse is reserved by nature for a lifelong, exclusive, male/female relationship for the begetting and raising of children, and the mutual care of the spouses. This is intuitively obvious and proven by common experience. Therefore, those who reserve intercourse for marriage are happier than those who don't; just as those who eat as nature intends (in moderation, primarily for nutrition and growth) are happier than those who abuse food (who overeat, eating primarily for pleasure).
Stating that two plus two equals five does not make it true. I reject that "nature" reserves sexual intercourse for any such situation as you aver. Were that true, either there would be no time at which pregnancy was improbable or humans would be disinterested in sexual activity at such times. Additionally, if what you aver is "reserved by nature" were indeed reserved by nature, humans would have no interest in intercourse if they were not in a monogamous, committed relationship. Again, as the predicate of your proof is under dispute, your conclusion is as well.
OK, then find the fallacy in the following proof:
1) Children are best raised by their natural parents who have committed themselves to each other for life. We know this through intuition, empirical evidence, normatively (historically) and naturally (the nature of parents and parenting).
2) Nature intends children to be raised by their natural parents who have committed themselves to each other for life, since this is best for children.
3) Nature designed intercourse, which brings children into the world, to serve the end of #2, i.e., to solidify the lifelong commitment of the parents for the good of the children.
4) So intercourse serves two ends: procreation and the unity of the couple.
5) Any sexual act outside of marriage is unnatural and disordered.
Were that true, either there would be no time at which pregnancy was improbable or humans would be disinterested in sexual activity at such times.
In such periods intercourse serves the unitive purpose, which is in the best interests of parents and children.
Additionally, if what you aver is "reserved by nature" were indeed reserved by nature, humans would have no interest in intercourse if they were not in a monogamous, committed relationship.
An analogy might be helpful. Did nature intend us to ingest good-tasting, poisonous substances, since our nutritive appetite desires them? Or did nature give us intelligence to use in order to avoid ingesting good-tasting poisons that appeal to our natural, nutritive appetite? Nature intended our nobler powers to guide our lower powers for the greater good of our entire being.
In fact, there is a mixture of truth and falsity in your argument. The truth is that before the Fall our lesser faculties corresponded with our higher faculties, and the intuition that our powers are partially disordered is correct.