I can’t find the relevant law in british-columbia but it will be similar to most other common law jurisdictions so let’s look at new-south-wales.
Section 61HC of the Crimes Act 1900 says:
61HC Meaning of “sexual act”
(1) For the purposes of this Division, sexual act means an act (other than sexual touching) carried out in circumstances where a reasonable person would consider the act to be sexual.
(2) The matters to be taken into account in deciding whether a reasonable person would consider an act to be sexual include—
(a) whether the area of the body involved in the act is a person’s genital area or anal area or (in the case of a female person, or transgender or intersex person identifying as female) the person’s breasts, whether or not the breasts are sexually developed, or
(b) whether the person carrying out the act does so for the purpose of obtaining sexual arousal or sexual gratification, or
(c) whether any other aspect of the act (including the circumstances in which it is carried out) makes it sexual.
(3) An act carried out for genuine medical or hygienic purposes is not a sexual act.
A reasonable person in the context of a “kink community” would consider BDSM to be sexual under (2)(b) & (c) irrespective if there was involvement of genitalia, anal regions or breasts. However, discussion of BDSM in the context of a health class within a high school curriculum is not a sexual act because a reasonable person would not consider it one
Note that a “sexual act” is different from “sexual touching” or “sexual intercourse” and the particular circumstances determine whether any of them is a crime in any given situation. A sexual act involving children (under 16) is a crime.