unpa's semantic space has to do with sex and sexual activity. The number of participants doesn't matter; solo masturbation is equally as valid to describe as unpa as an orgy, as is anything between. Kink can be a part of unpa, but it is specifically framed as a part of sex, whether in the lead-up as in foreplay, in the act, or any other part of sex. For comparison, horniness and other feelings or concepts related to sex are generally not called unpa, but they are often considered in their relationship to sex. Additionally, sexual positions and sexual preferences are generally not unpa, as they are not the act. However, they may be framed in their relationship to sex, such as by use of the word unpa as a modifier.
Gender and biological sex do not play a role in unpa's semantic space; put another way, unpa doesn't change when the identities of those involved change. It is still unpa.
Reproduction also generally does not play a role in unpa's semantic space, a feature it shares with the word "sex" describing the act but not reproduction. Reproduction may be referenced in relation to unpa, such as when discussing breeding animals, but unpa generally does not imply or refer to reproduction on its own. There are caveats to this however; some speakers will use unpa to refer to pollination, one of the means by which plants reproduce. By the use of unpa, this is being framed as a sexual act at the same time as an act of reproduction; this is by analogy to sex, as pollination is the closest similar thing that applies to plants.