It depends on the situation. If I was there, it would probably be fine because they're not being sneaky. They'd have asked if it's okay. If I was not around and suddenly pictures of it appeared on facebook, I'd question their sexuality and be a bit worried. I mean, if they make out with someone of the same sex without telling me, who's to say they're not making out with someone of the opposite sex too?
I am inclined to question the straight partner's "straightness" if they make out with someone of the same sex ... i mean, a straight guy just does not make out with another guy and i don't think a straight girl does either ... but that aside, in my opinion, if you're in a committed relationship, any intimate contact with anyone else, regardless of their gender is, for me at least, cheating ... unless it is something planned with and known about and okay with your partner.
I guess I'm a bit unusual in thinking that making out doesn't have to be cheating at all. If it's just a bit of silly fun at a party, then why not? It's different if the partner has a romantic interest in the other person, or they do something like... go by themselves to make out on a bed? See the difference, or is it just me?
I think your use of 'make out' to me would definitely be cheating. However, I did once peck a female friend of mine on the lips because she was moaning about how she'd turn 20 having not done anything even mildly exciting and having never kissed a girl. I did it as a joke though, so my boyfriend didn't care. If I'd been out clubbing or something and started making out with a girl I think that's entirely different and would definitely count as cheating!!