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 on Tumblr show how one man does it with a dildo up his ass and some careful athletics:

Hands-Free Masturbation And Ejaculation

(spare Tumblr link in case the first one kablooies)