I love the awfulness of this. One of my favourite fanon/canon things is that the Marauders' trust issues dissolved during the First War because of sex issues. :) So this is just perfect. I love how calm and cool James is here, and how hot-headed Sirius is. And poor Remus! :)
And poor Peter, heh. What an eyeful, when you'd never seen any of that among your friends (or anyone!) before. :)