A lot of the problematic responses seem to be based on some wrong assumptions, aside from the fact that some of them seem to imply that men are entitled to having women eat their M&Ms regardless, or that women are responsible for the M&Ms being poisonous:
The metaphor isn't talking about women being friendly to men, or having sex in the context of a relationship where people know each other pretty well. It's specifically talking about women putting themselves in vulnerable situations with a man they don't know well, where he could easily assault them if he chose to. I suppose you could make that about women turning you down for sex when you don't know them well, but unless that sort of sex is your aim or you're feeling rejected because a woman you don't know doesn't feel safe riding home with you or coming up to your apartment, then the metaphor isn't actually talking about anything that's going to make you feel fulfilled. Maybe you have to form relationships with women in safe places first before they'll come home with you; big deal.
The metaphor also isn't saying that women shouldn't talk to men or have sex with them. It's descriptive, not prescriptive -- and it turns out that, descriptively, most women have sexual relationships with men regardless of poisoned M&Ms, and the vast majority of women are friendly to men. Many women even end up getting into vulnerable situations with men they don't know well, often for pretty strong reasons. What happens is that women mostly do these things with a bit of attention to the potential threats.
(Also, most women do not individually have sex with the vast majority of men who might ask, but that's not about poisoned M&Ms; that's about people being picky about sex partners in ways that are far more complex than value judgments.)
And there's the thing that the metaphor, really, is kind of broken. This should be no surprise; all metaphors are broken -- they explain the thing they're meant to explain, and they fail at the edges where they stop mapping to reality. So, if you're going to have a meaningful conversation with a metaphor, either you have to take it on its own terms or talk about where it doesn't apply. This metaphor is about why a few men being dangerous means most women quite reasonably view all men as potentially dangerous even though most men aren't. It's not about what women do with that view; if I had the bowl of M&Ms in question, I'd throw it out without a second thought (even if I had a poison-test kit!), and that obviously doesn't map to what most women do with men. And it's not about the numbers, either; 1 in 4 risky interactions with men don't end in assault even given 1 in 4 men will assault a woman at some point in their lives. But neither of those is the point of the metaphor, and if that's your objection, the useful way to say that is not to say "but you should eat the M&Ms anyway."
The metaphor also leaves out something that I think is really important, because it's focused on the poison M&Ms -- the interactions with men that leave a woman assaulted or worse. The claim is that the rest of the M&Ms, the vast majority of them, are just fine. The thing I've been realizing, listening to my friends talk about this (and the post I linked to above by metaphortunate is a good example) is that mostly what happens when a woman turns down a man's offer of a ride or invitation up to his apartment or whatever because she doesn't want to take that risk right then, is that he either takes it personally or gets overly apologetic and in any case it becomes this big deal with a lot of emotions and becomes this long-lasting awkward thing. And, no, that's not a "poison M&M" that gets her assaulted, but it's not anything close to "just fine" either. And it's not 1 in 4; it's "most of the time." And one of the problematic things about a lot of the responses is that they're directly part of this pattern of men hearing something like a "no", even when it's not personally directed at them, and making it emotionally painful for the woman saying it.
One of the many reasons that side of things is important is that ... well, it's hard to see where I can personally do a lot about men who assault women. Men who think that's okay tend to be men I avoid associating with, and the public persuasive essay has never been a thing I'm good at. But men who get all feelings-hurt about perceived rejections from women? It's a lot easier to find something useful to do about that: It hurts to admit it, and it's something I really don't like about myself, but I've been one of those men a few times. And so I can start by learning how to not do that again.
Crossposted from Dreamwidth (original here), with comments. Comment here or there; comments here will eventually be duplicated to there.