Does "having sex" mean possessing gender or engaging in sex acts?

What each sexual act means to you personally—an expression of affection, a recreational event, the blending of your essence with your one and only—is not what I intend to discuss, although that's a fascinating subject I may address some other time. I want to inquire into the meaning of sex as a word, and what acts constitutes sex.

If an English speaker says the word "tree" to another English speaker, both get the idea of a piece of vegetation that has a trunk and leaves and can grow tall, although one may be picturing a fir while the other a weeping willow. Further conversation will align their images better so that they both understand they are discussing the same object.

Sex, it is a simple three letter word. It does have several meanings, however. Gender is the usual synonym, although that can be ambiguous too since masculinity and femininity are also frequently given. "Having sex" makes it even more uncertain. Does that mean possessing a gender or does it mean engaging in a sexual act. If the latter meaning is intended, of what does an act of sex consist?

If a sex act involves the genitals of only one of the participants is that sex? If no genitals of anyone are being employed but the act involved is arousing and potentially orgasmic, can it still be considered a sex act?

The definition of what sex is was brought up for extensive public discussion fifteen or so years ago during the administration of President Clinton. No commonly agreed upon solution was found then or since.

What brought this all up for me was the report of a recent study I read online. The gist of it was that among Americans 65 or older those who had sex in the last month were happier in their lives and their relationships than those who had not had sex within the past year. Nothing was said—at least in the précis I read—about what having sex consisted of, whether the sex was enjoyed with a regular partner or another, or whether, in fact, the sex was enjoyed or endured. A whole lot I would have wanted to know in order to make sense of the findings was omitted.

A further brief note on this summary report stated "Sex was defined as oral, anal or vaginal." "But what about manual?" was my immediate response. We're talking about seniors over the age of 65 and of all sexual orientations (I suppose). For those with body function difficulties and those for whom an erect penis was unnecessary and/or unavailable using one's hands (or two's hands!) is a logical way of giving and receiving satisfaction. Why was that omitted? Or is it possible that the authors of the study or the reporter covering it for the online news service where I read about it did not consider manual stimulation as an act of sex. And if so, why not?

So here we are back to the question I originally posed. Put more directly, if petting for the sake of giving and receiving sensual and sexual pleasure is not considered sex somebody sure is missing out on something good.