You wait until your breathing returns to normal before you pull out of James. Your cock, still half-hard, takes an erotic ammount of time to pull out, all of it's incredible twelve inches. Once you finally manage the withdrawl of your cock, the torrent of cum you shot into James begins to run out of his gaping ass.
You climb off the bed and swager over to the mirror on the wall, loving the feel of your giant cock swinging between your legs, almost bumping into your kneecaps. "Man, this body is incredible, thank you so much James" you say to him as you flex in front of the mirror, admiring your new form. "Yeah its -pant- really incredible -pant-" James says as he gasp for air, still recovering from the incredible sex "But there are some -pant- things you still need to -pant- know..."
"What more do I need to know about this hot body" you say, admiring the incredible way your biceps look when you flex them. You gaze loving at your body and are about to begin another comment about how hot you look when suddenly a terrible sight occurs. One of your huge biceps begins to deflate and melt away, as if you were an inflatable toy with a hole in it.
"James, what the fuck is happening?" you cry out, your eyes glued on your rapidly reversing body. Your muscles are diminishing everywhere on your body, and finally you see your huge, deadly cock begins to shrink, losing inches all over until it returns to it's original, below average size.
"Yeah buddy, that was one of the things I meant to tell you. The magic isn't permanent, once you cum the magic ends and you'll revert soon after" he explains, taking your transformation back rather normally, though you note the disappointment in his eyes.
"But, but I jerked off before I came here and nothing happened!" you cry, running your hands over your old, completely uninteresting (in your eyes now) body. Your hand finds it's way down to your cock, which seems absolutely pathetic now that you've experienced what it's like to have a giant cock.
"That's because you put the trunks on right after you came" he explains "They renewed the magic before it had time to run out"