I'd say "to mate" is mostly used for animals. You can use it in reference to people, but to me, it sounds either detached and clinical (as if you were referring to people as some sort of livestock or wildlife), goofy (as if you wanted to use a stronger word than "sex" but wouldn't let yourself go for the f-word), or insulting (as if you wanted to refer to someone's sex partner as a beast of some kind).

In Klingon, from the example of {yIH ngaghwI'}, it seems that you can use {ngagh} to refer to people, but it probably has more vulgar overtones than {nga'chuq}. (For instance, you might use it in the chorus of {Sumqu'} by {vI' jav jav 'uj 'abbogh Hut'Inmey'e'}: {qangagh vIneH, Ha'DIbaH vIrur vIneH.})