When You Start Dating Someone, Can You Still Sleep With The Ex Until You've Had The "Talk"?S

Our dear friend Spencer went to see Forgetting Sarah Marshall over the weekend in search of levity, but he left the theater with a heavy conscience and thought you guys might be able to help. Apparently, the plot of the movie concerns that tricky period when you've started dating someone for whom feelings are mutual whilst remaining hung up on someone for whom your feelings are unrequited. New Person and you have not yet had that critical "So..." conversation but you feel sufficiently distracted by the New Person that the Ex is suddenly once again attracted to all your new non-desperate pheromones. Can you sleep with Ex? Why would you want to? And if you do, do you say anything to New Person about it? We know the answers, but Spencer wanted some second opinions. His predicament, after the jump.

Show of hands: who saw Forgetting Sarah Marshall this weekend? Much as you might yearn for an Apatow backlash, you'll find no pretext for it in the movie. But you will find a pretext for pondering the relationship predicaments of men who mean well. Or at least who flatter themselves that they mean well while actually just fucking everything up like a typical douchebag. Like me.
["Flattering themselves into thinking they mean well" = pretty much the definition of asshole. -Moe]
Case in point is something that occurred in FSM that, once upon a time, happened to me. (What follows might be a bit of a spoiler, but I'll minimize the damage.) Guy thinks New Girl is pretty great, and there's something between the two of them. Who knows where it could lead! It's all so new and unexpected and exciting. The only thing is is that when one of these things starts, it's not as if the past ceases to exist. And let's say the Guy has a fraught and complicated and never-ending quasi-relationship with someone who keeps on breaking his heart but he just keeps going back because he doesn't respect himself. Let's further say that he sleeps with Heartbreaker a couple days after he's begun seeing New Girl, although there is no expectation of monogamy, let alone an actual discussion of Whatever This Is (And Could Be!) between Guy and New Girl.
[You should be too busy fucking New Girl to ponder Heartbreaker, except in the vaguest, fuzziest "God, I was a loser" sort of way. Fuck the "discussion." Fuck more. That is all. -Moe]
But Guy has this feeling that if there is, in fact, going to be something with New Girl he had better do the honorable thing and disclose what happened. Yes, yes, some say, the really honorable thing would have been not to have slept with Heartbreaker, but you know what? Fuck you, because I don't have a time machine, and have you never done something like that? Really? You don't understand what it's like to have difficult-to-resolve feelings for someone, even though you know it's probably futile, and why do you expect that "moving on" is a linear progression?
[Actually, I'm pretty sure I understand difficult-to-resolve feelings for someone who keeps constantly dumping you and as I understand them, they are pretty much entirely rooted in insecurity, which you can definitely suppress by accepting that you are highly flawed and barely worthy of love, and fucking a lot, but carry on. -Moe]
So let's finally say that the disclosure made a whole lot more sense in Guy's mind than in practice. Guy's female brain-trust is split down the middle about whether disclosure was the right call: half say that before there's a discussion of monogamy, you keep all that other-women shit to yourself; and the other half say that it'll hurt a lot worse if something actually develops between Guy and New Girl and New Girl later found out about Heartbreaker through the seemingly-inevitable run-in or third-party chatter or whatever. (Interestingly, the mix gets scrambled when female brain-trust members are asked if, put in this situation themselves, they'd want to know.)
[NOOOOOOo no no no. Why the fuck would you tell her? Who cares? She's not your wife! You're only trying to diminish her expectations for trustworthy behavior in the future. This will fuck everything up permanently. But hey! — that's what you wanted. Because you're UNWORTHY OF LOVE. So you lose, but really you win. Congrats! -Moe]
I open it up to you, womanity: What should I have done here? I didn't mean to do harm in this case, and while the seeds of Crap Behavior lurk in all of our hearts, oftentimes marginal-to-harmful behavior is attributable to confusion, not malice. Remember, not-sleeping-with-Heartbreaker is not an option, so I don't want to see that popping up in comments. You make the call.