One of the things that commonly trips people up when talking about natural law theory is a failure to distinguish laws that impose on individuals from laws that impose on species. Thomas Aquinas has a good example of the distinction when he discusses manual labor (which, it must be understood, is a somewhat broader notion than what we usually call 'manual labor', since it includes almost any sort of working with the hands). There is a natural-law imperative to engage in manual labor for the purpose of sustenance (food, primarily, but also shelter, clothes, and the like). But this is a species-level law. In other words, as a matter of practical reason (which natural law always is), it is imperative that human beings do the manual work necessary to sustain themselves. But it does not follow from this that everyone ought to do manual labor for this purpose. All that follows is that individual human beings should not be avoiding manual labor for bad reasons, because it's an important thing that needs to be done. But it's perfectly possible that particular people will have good reasons for avoiding it in favor of something else. And indeed there are good reasons for particular people to avoid it. For one thing, it takes up time, and there are worthwhile but time-consuming non-manual activities that need to be done (like teaching). Thus it makes much more sense for teachers, for instance, to be paid for their work, and thus to provide for their sustenance, and the sustenance of others, in that way. So it's a matter of natural law that manual labor should be done for sustaining human beings; but all that means is that someone should be doing it, and those who are not should have good reasons for not doing it at least to some extent, even if it's as small as a bit of gardening or handicraft or volunteer work.
The distinction turns out to be utterly crucial for sexual matters, and there is a bad tendency on the part of some people who appeal to natural law to overlook the distinction when it's convenient for them. It's a matter of natural law that human beings should engage in sex for the purpose of procreation. But this, again, is a law imposing on the species, not on individuals. It does not follow from this that any particular person finds himself or herself imposed upon by this law to be one of the ones who are sexually active for the purposes of procreation. It can be entirely reasonable for people not to engage in any sex at all (e.g., if they are celibate in order to devote themselves to other important things), and it can be entirely reasonable for particular people to be sexually active without reproduction in view at all. There is a danger of thinking that every single sexual act must have reproduction as its particular end; when in fact, reproduction is simply one the common ends of sexual acts in general, something to which they tend as a whole, ceteris paribus. Because sexual activity for reproductive purposes is a matter of practical reason and natural law, it's not a matter of indifference; but much of what natural law will have to say about sexual matters for the individual will be very oblique, at least as far as reproduction is concerned. This means that moral problems raised with regard to sex and reproduction will be much murkier than one might expect, requiring a great deal more hard reasoning to locate within the whole scheme of natural law. But this is as it is; it does no one any good to try to short-circuit the whole work of practical reason by trying to fit all of sexual ethics into the aspect of natural law devoted to reproduction.
