Is "Everything that leads to the unlawful is unlawful" a sound legal maxim, and does disliking this maxim lead to apostasy?

The principle "whatever leads to the forbidden is forbidden" is correct, but a means may be subject to something that removes the ruling of prohibition from it, if it is a means to permissible acts that are stronger and more numerous than the forbidden acts it leads to. In such a case, the stronger takes precedence. The means to a forbidden act may also not be forbidden if it leads to a preponderant benefit, such as the mixing of men and women in one country, despite it being a means to zina (unlawful sexual intercourse), because the cooperation of both genders in various fields of life is a preponderant benefit.

Permissible acts that lead to corruption are of three degrees: 1. Those whose leading to corruption is rare: Their ruling is permissibility, such as cultivating grapes, a man teaching women, and women going out for their needs. Here, benefits and harms are weighed, and if the benefit is preponderant, it is not prevented. 2. Those whose leading to corruption is frequent and common: They are prevented as a pre-emptive measure (sadd al-dhari'a), such as selling weapons during times of strife, or leasing property to someone who will use it for disobedience. This is a temporary prohibition. 3. That which the mukallaf (accountable person) uses as a stratagem to legitimize the forbidden: Such as using a trick to engage in usury through Bay' al-Inah (a type of sale that leads to interest), which is forbidden by clear text.

Accordingly, permissible acts that lead to some forbidden acts do not lose their ruling merely for that reason. Rather, the corruption must be stronger and outweigh the benefit. A person's attention to their appearance is permissible, even required, and it has purposes and benefits that are not outweighed by the mere possibility of women being tempted by him. As for what happened to you, it is neither apostasy nor a denial of a known necessity of religion. Rather, it is ignorance that led to a distortion in perception, and you are excused in this.