It is not a loop hole but it is a type of birth control – specifically, it is the type of birth control that respects and utilizes the manner in which God created us.
Condoms introduce a foreign element into sex and deliberately and actively blocks the reproductive means.
Contraceptive pills introduce a foreign element and also deliberately and actively block reproduction.
Withdrawal introduces a foreign element and deliberately and actively blocks reproduction.
Natural family planning utilizes the natural infertility of a woman’s cycle and inactively avoids reproduction.
The basic framework of NFP, that of tracking a woman’s cycle, can and should be practiced at all times by a couple. Using the knowledge of that cycle to abstain from relations during the fertile part of her cycle should only be used when necessary.
The main difference between NFP and other methods of birth control is that NFP is built-in to the fertility cycle. A woman is naturally infertile because God made her so. NFP utilizes this natural infertility.
Other forms of birth control actively work to prevent and pervert a natural part of sex. Condoms and diaphragms block an ejaculation. Birth control pills alter a woman’s hormones to stop ovulation. Etc.
It basically comes down to this: the ends do not justify the means.
Are there legitimate reasons to avoid a pregnancy? Yes.
Are there legitimate means by which to avoid pregnancy? Yes. Abstain from sex. That is exactly how NFP works. You do not have sex when a woman is most likely to conceive.