I don’t think it’s fair to compare NFP to moderate eating. The latter is essential to good health, while the former isn’t essential to sexual discipline (in the self-control sense, obviously). It can certainly be a useful tool, but given that it hasn’t existed for more than 60 years or so, it’s not reasonable to assume that every couple until the 1950s or thereabouts had a lack of sexual self-control.
NFP is a tool, nothing more, nothing less. Some tools are good in some situations, while others aren’t. A woman who really needs to avoid having children but who hasn’t had any success in that regard with any (or any combinations) of the NFP methods doesn’t need NFP; she needs to abstain. On the other hand, a family who doesn’t have just reasons to avoid, or whose reasons are on the fence between just and “meh,” might not need NFP, and may find that extended breastfeeding or some such works well for them, or they might actually need it; only they can decide. Yet another family might find NFP to be an effective tool for postponing or avoiding children, or for sexual self-control, or even as a sort of penance (ie, abstaining from sex for a time by mutual consent as per St. Paul).
Likewise, making one decision usually involves sacrificing the potential for another decision. That’s not always a bad thing, but it is realistic. So a family might decide to go ahead and conceive another child, but in doing so, they know that this means postponing some trip or expense as far as possible. A family might decide to avoid another child for a time, and accept the sexual distance that NFP does bring to a lot of marriages. (A couple I know who practice NFP for a life-or-death reason often have to abstain for 3+ months at a time due to wonky cycles and work scheduling, which is really, really hard on a marriage.) Or a family might decide that that distancing isn’t good for their particular marriage, and knowing that they don’t have a reason that makes avoidance really necessary, say “whatever happens, happens, and that’s okay, but for us, sex is important enough in our marriage that having it more often is worth the sacrifices of another kid.” And that’s also okay.
In short, the idea that NFP is necessary in all marriages is really flawed. It’s a tool, it works well in certain areas, but it doesn’t in others…and that’s okay!