The act of making out with someone's wife or girlfriend. Is that also sexual misconduct?
Obviously it is the source of possibly disharmony, to say the least. As for sexual misconduct, with a wife of someone else, yes, I think so, according to AN 10.176.
Sexual misconduct in Pali is kāmesumicchācāra, cāra - doing, micchā - wrong, kāma - which refers to either sensual pleasure in any of the 6 senses, or sensual desire. So if one is acting out of sensual desire, which I can't see how one isn't if they are kissing, with someone who is designated as an object of kāmesumicchācāra, then tathārūpāsu cārittaṃ āpajjitā hoti - he falls into such a manner of acting.
Those who are objects of kāmesumicchācāra are a woman protected by mother, father, both parents, brother, sister, relatives, clan, (all presumably implying that she is young in age) or the same nature (presumably meaning incest). Also, one who has a husband, is liable to punishment, or on whom a garland has been placed (engaged).
I am not sure that they had the notion of "girlfriend" in the Buddha's time. Even until earlier in the 20th century, if one was seeing someone frequently, as one sees a girlfriend, it would be presumed to be some form of courtship, and one would be expected to remain celibate until marriage. So obviously social convention changes, and it seems the Buddha was simply proscribing rules against acting in a way which most in society then, and still today, would have considered sexual misconduct. But today, I do not think people expect you to kiss another's girlfriend, and I would presume it is still against social convention - but I have no idea, maybe you live in California or something.
I recall reading somewhere that even giving a flower to someone else's lover is sexual misconduct. I forgot where I read that though.
This may be some kind of interpolation of "antamaso mālāguḷaparikkhittāpi," even one on whom garlands have been hung.