I found this blog post fascinating:
It turns out that different people have very different definitions, and they make all kinds of interesting distinctions. For example, some people only think that intercourse “counts” as sex if they have an orgasm. Further complicating matters is the fact that who’s participating in a given behavior influences what counts. Specifically, we seem to hold ourselves to different standards compared to other people.
For example, several studies have shown that people are more likely to label a given behavior as sex to the extent that their significant other did it as opposed to themselves. In a study of 839 college students (96% heterosexual) who were asked whether oral contact with another person’s genitals counted as sex, it turned out that just 36% of women and 39% of men said it did when they imagined themselves doing it . However, when asked to imagine their partner doing the same thing with someone else, 62% of women and 63% of men suddenly viewed it as sex.