I had creampied girlfriends in the past, but I had never seen one myself until I participated in a MFM.
I was taking a break, watching my friends go at it. He was on top of her, pinning her arms with one of his hands. He told her he was about to come, and began to withdraw, but she told him to come in her. He thrust into her as far as he was able (which left an inch or so of his cock visible) and began to come. It was incredibly erotic to see his cock, ass, and thighs pulse and thrust into her while she squirmed under him, legs splayed.
To this day, it is one of the most beautiful and powerful expressions of sexual energy I have ever seen.