seksumi

This is the neutral word, and that is the point of the page. Seksumi is the standard, unmarked verb every adult dictionary of Esperanto carries for having sex. It is neither clinical nor coarse: it is what a doctor, a textbook, a novel and a friend would all use, in the way English "have sex" is usable in all four settings while its alternatives are not. Esperanto does have blunter options, as every language does, and the reason you do not need them listed here is that seksumi covers the ground.

It is also the best argument for -um- on the site. The regular suffixes compute: -il- is a tool, -ej- a place, -ig- to cause. None of them would produce this. Seksigi would be to cause sex, whatever that means; seksejo a place for sex; seksa is just the adjective, sexual. What was wanted was a verb meaning "to do the thing that has to do with sex", and that is precisely the job description of -um-: it marks the word as related to its root and refuses to say how. A century of use has fixed the meaning, which is why seksumi is vocabulary you learn rather than a derivation you compute.

Around it. Amoro is lovemaking as a noun, warmer and more literary; amori is its verb and is the usual choice in poetry and romantic fiction. Seksumado is the activity as a noun, seksumado sen protekto unprotected sex. Sekso itself covers both sex and gender, and the tangle that causes - the -in- suffix, ge-, the proposed pronoun ri - is the subject of gender in Esperanto.

A note on register when you write. Because the verb is neutral, it takes its colour entirely from the sentence around it. In a health leaflet it reads as clinical, in a novel as plain, and in a joke as a joke. That is a useful property and a normal one; it is the same property English "sleep with" does not have.