The abbreviation "MM" stands for "man man", i.e. for two men.
"MM" means "man man" and accordingly stands for two men. How or how intensively the two men are then connected to each other (and what they do with each other) is not yet indicated by the "MM".
What is certain, however, is that it is often used in chats to explain sexual practices in a little more detail. A classic example: the Threesome or a gang bang adventure. Many gays (and of course straight men) are of course interested in how the corresponding constellations should be put together before they set out on a date.
A gay man is killed at a Threesome with two women are unlikely to find much pleasure. However, as soon as another man joins the group, sex should become more interesting again for the gay.
However, "MM" can also mean a classic sex date between two gay men. Many chat sites have set up a separate area for "MM" lovers, in which they - knowing full well that the other person is also gay (or bi) - can approach each other undisturbed.
Sometimes, however, the corresponding abbreviation can also be found in the description of sex toys. It then usually indicates that the relevant ToyFor example, a double anal dildo can be used by two men at the same time.
However, the abbreviation "MM" - as already mentioned - only says "man man". What exactly is hidden behind it can only be clarified in the complete context. So if you want to know more, you always have to consider the context. Thus, there is (usually) nothing standing in the way of a horny sex experience with at least two gays.