You gentlemen will appreciate how rare it is to ejaculate (while awake) without any stimulation of the penis. But it is possible, especially when the prostate is being pressed upon properly. This pair of animated .gifs show how one man does it with a dildo up his ass and some careful athletics:

Hands-Free Masturbation And Ejaculation

2019 update: This link originally went to Tumblr, and broke in the Great Tumblr Pornocalypse Fire of 2018. Fortunately, the Internet Archive’s WayBack Machine had preserved a copy.