Of course, there is another option—a truly devious master plan—but I'm ashamed to suggest it. In fact, I'm ashamed on behalf of all men for even allowing the idea to gain currency in my mind, because if I thought of this, then I know many other men have too, and, well, that's just despicable. But here it is (forgive me): Go on the trip, have fun, have sex, act loving, come home, wait a month (or, preferably, two), then tell your girlfriend you've fallen in love with someone else. Granted, this route would require an Oscar-caliber performance (not to mention an extremely patient new girlfriend). But if you could pull it off, it might preserve the trip as a wonderful (if still technically fraudulent) experience for your soon-to-be ex.
All of which, I hate to say, raises the question: Who says Flame #2 will stand by as you go off to South America with Flame #1? Doesn't she have an opinion in this little soap opera? Don't you think she might be blessed/cursed with the same wandering eye as you? Which raises another question: How can men be such liars and so trusting simultaneously? Talk about the gift of ignorance!
Cathi: Not telling your girlfriend before you go isn't fair to her, the new person, or—say it with me—yourself. And having been in the same hiking boots at one point, I have to ask why you'd want to go on a trip with one girl when you're itching to be with another. (OK, the mountains, the spiritual thing … but still.) Then again, I can't imagine how you can be with her now while feeling this way. Maybe it's just me—bad poker face, and all that.
Anyway, how generous is it, really, to let her get that much more bonded with you and then tell her, "Sorry," but you've actually been in love with someone else for months now? (I know, you wouldn't tell her that part, but, believe me, she'd find out.) She'll be hurt either way, but if you wait, she'll also be enraged at having played the fool for so long. As for the trip, unless you're a master of deception, she'll sense something's changed between you and won't know what or why. Nothing like that for a scenery wrecker.