A colloquial term on the internet meaning "mansex" or homosexual male sexual relations. It comes from the term "mansechs", which was (is) a popular way of spelling "mansex" on many internet forums and that bastion of the linguistically adept, IRC.
Him? Oh, he doesn't like women anymore. He has given in to the dark force that is teh manches.
Sperm, semen, ejaculate, cum, jizz... what have you, manch is what some people like to refer to their penile secretions, especially when said fluids resemble the color and consistency of ranch salad dressing.
Hey baby, you look like you could use some manch dressing on that pube salad!