male g-spot

Does it exist?

Supposedly, it can be found about 4 centimeters inside the rectum, toward the belly side.
The same people who say it exists also say that the best way to stimulate it is thus:
The man lies on his back, legs apart and bent at 90 degrees. His partner inserts a finger (palm up) and stimulates the G-spot by 'come here' motions with the finger.

If done correctly, the man should come here.

This description is somewhat misleading. The stimulation occurs not because of an actual spot inside the rectum, but because pressure is being applied to the prostate. The same procedure is used to check for prostate cancer in men, but done correctly can also cause an aroused man to ejaculate forcefully. For best results, combine this stimulation with direct oral, anal, or vaginal stimulation of the penis, rather relying on applying pressure to the prostate alone. With some practice, this is easily achievable by two people, but this can often be a good excuse to get a third party to "lend a hand" as it were.

It's important to note that this method of prostate stimulation takes getting used to, and can result in soreness the first few times.