John had a abusive boyfriend in the past.
I remember John had only dated on man before back when he was in uni and the man was a abusive jerk but John didn't realize but swore off men none the less. Years later Sherlock convinces John to be in a relationship with him but promises to take it as slow as John wants since he's skittish about any relationship with men that goes beyond flirting. I think Sherlock tries to get John to file a report but gets shot down and he doesn't even tell Sherlock the man's name saying it's in the past.