Marshmallows won't "melt" at body temperature. They are composed mostly of table sugar and gelatin, whipped up into a fluff and hardened.
It takes a lot of heat to "melt" them. When inserted, the moisture in your rectum (or vagina) slowly dissolves them. Once they start to dissolve, they release the sugar, which really irritates your passage, causing more fluid to be drawn in which dissolves more marshmallow, and so on, until the urge to poo becomes overwhelming.
So, while they may take quite a while to start working if your passage is dry, once things get going, the laxative effect can increase quite rapidly and they become completely impossible to hold. A very urgent, slick, sticky, messy, poo soon follows, often followed by a few more in the next couple of hours.
Sugars in the colon are powerfully laxative, whether they get there by passing through your system undigested (Lactulose, Sorbitol, Fructose for some people) or by suppository or enema (molasses, honey, Mayo, glycerin).
If you don't get an effect after a half hour, drink a glass or two of water and go for a walk. You might want to wear a diaper.