The boundary line between what is considered sex and what is not is very thin, and is defined in different ways by different people. In this article by Greta Christina, the female character that is portrayed goes through life counting the number of times she has had sex, but encounters numerous accounts or incidents where she is baffled by what is considered sex or just foreplay or being sexual.
For her and most people, sex is defined as penile-vaginal intercourse, or coitus.
But there is also that very shady gray area where everything sexual has occurred except for the penile-vaginal intercourse. Is this considered sex or just really good foreplay? Who is the one to make that decision, for it is different for each person depending on what kind of morals and values that person possesses.
In the article, the female lost track of the number of times she had sex because she had a difficult time distinguishing what was sex and what was not, and if she did come to terms of what she considered was sex, she would have to think back to all those times where she might have had sex and redefine those occurrences.
Then lesbian sex came into play in her life and blew her definitions out of context. Lesbian sex was a whole new perspective and forced her to totally think about what was considered sex and what wasn't. In lesbian sex there is no penis involved so the penile-vaginal intercourse guideline was no longer an option.
In lesbian sex there are different ways such as touching and kissing, licking, rubbing and grinding against each other, etc. She would once again have to go back and redefine her guidelines on what was sex and what was not.