Although there may be other posts like this one out on the Internet, I've been thinking about this topic for years and finally just wanted to post about it. This post is about the semantics of the term "pre-marital sex" and differentiating it from other forms of non-marital sex.
Basically, there are three different categories of sex when it comes to marriage: 1) Marital sex - that is, sex within a marriage; 2) Extra-Marital sex - that is, sex by one marital partner with someone outside of their marriage; and 3) Non-Marital sex - that is sex by someone who is not married. While categories 1 and 2 need no further breaking down, category 3 has some breaking down to be done (it may be noted that extra-marital sex could be included within the non-marital sex category, I'm going to let it remain separate).
Non-marital sex is simply sex by an unmarried person, whether they had formerly been married (you could call this post-marital sex (whether it's with their former spouse (in cases of divorce (as opposed to being widowed (and, no, I'm not touching necrophilia))) or with someone else)) or not otherwise married. I hesitate to say not yet married because it presumes one will get married at some point. This problem remains with the term "pre-marital" sex, unless, of course, one is engaged. In such a case, one has a marriage on the horizon and knows they are to get married and has sex with their fiance. This is the only time that the term Pre-Marital sex makes any sense - they are having sex before their marriage. Otherwise, it is overly presumptuous that this person will marry. Whether they will die before getting the chance to get married, they have no desire to get married, or simply are forever unmarried.
I don't know if these distinctions amongst non-marital sex sub-categorizations will ever catch on in a widespread sense, but I do hope people think more intelligently about using the term "pre-marital sex".