I had never heard thus before, so I looked it up. Honestly, the story seems pretty poorly sourced. McConnell was officially discharged for medical reasons. The accusation seems to hinge on the testimony of a solider named Alan Lynch from Kentucky (I can't find any record of this Mr. Lynch outside of this story, which is suspicious) who overheard a Major mention that McConnell had committed sodomy in the barracks and would be discharged, and that McConnell's connections covered it up.
I lean very far left, but this doesn't add up to me. The story originated on Twitter (since deleted) and then circulated among some left-leaning tabloid sites trying to paint McConnell as a hypocrite and a closeted homosexual. Personally, I don't buy it.