As defined, {nga'chuq} means "sex (i.e., perform sex; always subject)"
according to HQ 1.3 p.9. I've always interpreted the "always subject"
annotation to mean that the parties named in the subject are having
sex with each other.
A discussion about {nga'chuq} came up on Facebook. Okrand confirmed this interpretation to me (that the subject of this verb must be plural, and are engaged in sexual intercourse with each other) in July 2013 when we had a brief chat about the Stonewall campaign. I'd made a note of it at the time in {boQwI'}, but don't seem to have posted about it to the mailing list, so I'm doing it now.