*update*
Well, at last I found a story that has Ennis sleeping with another man besides Jack (but it´s one of those hush hush stories, so I won´t mention the title). It was interesting to read, but very emotional for me. I don´t think I wanna read about that again. Once is enough. 
A hush-hush story? No such thing on this forum, Buffy. We don't have any restrictions on what stories can be discussed here.
And in original answer to your question, there are a number of stories that feature Ennis having a relationship with another man, almost all of them are stories that go past the original ending, i.e. after Jack's death.
My series of stories now called Greenlea Tales
Marge_Innavera's story "A Gift of Exile"
Tom Forster's "Ennis, A Sequel"
those are three off the top of my head.