At my school, all the freshmen do an anatomy project for health class. Each group gets a different organ system, and my group was 'lucky' enough to get the most 'popular': reproductive system. We in particular had the male reproductive system and it happened to be that we were all guys.
One Sunday all the guys (Eric, Tom,and Nick) came over to my house to work on the project. We started researching stuff, googling words like penis, testes, etc. Nick thought it would be funny if he switched it from a normal search to an image search, so all of a sudden we saw pics of penises and stuff like that. Then he took off the safe search and porn came up.
Being 14 year old guys we were all interested, so we looked for a bit. Nick (he was the instigator for everything) then said that to fully understand the male reproductive system we should see each other naked so that we could all see different types of penis (cut/uncut), scrotums, pubic hair, etc. We all eventually agreed. We began jerking off because we were all horny. Then we all shot our loads into an old coffee mug I had laying around. I mixed it up and globbed some onto the poster. We then covered the stain with a blurb about semen.