By TMZ

Former champion Kassim Ouma was arrested for battery in L.A. on Friday -- after knocking out a man who was allegedly making gay advances toward him ... TMZ Sports has learned.

Law enforcement sources tell us ... the former IBF junior middleweight champion is telling investigators ... he was hanging out in Hollywood early Friday morning when he struck up a conversation with a guy he just met.

Ouma says the two went back to the other man's home for a drink ... where the guy allegedly made a pass at the boxer.

We're told ... Ouma claims he politely declined multiple advances ... but says when he got up to leave, the guy made one final move -- and Ouma shoved him away.

The altercation then escalated ... with Ouma allegedly knocking the guy out cold.

Cops were called to the scene -- where the victim was treated and transported to a nearby hospital.

Ouma was ultimately arrested for felony battery causing great bodily harm.

We tried to contact Ouma for comment -- but he's still behind bars.