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When fecal matter and sperm mix, usually the byproduct of when a man ejaculates into another man's rectum via the anus canal. This is considered a gay sex act in certain circles.

In typical heterosexual relationships, a man's sperm usually fertilizes the women's eggs via vaginal intercourse. When it comes to making a "man butt baby," the egg is replaced with what is colloquially known as a turd.

"Taking a dump" or evacuating one's bowels after engaging in said act is also known as "delivering a man butt baby."
John and I made a man butt baby last night in WeHo!
by New Old Jersey May 02, 2015
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