When two men have seen each others penises,
"Me and Mark are Jungle Brothers now"
"How come?"
"I saw his penis so I pulled my pants down so he saw my penis too"
When two men have contacted the same STD from the same sexual partner.
“Yo, I heard you banged Adrian.”
“Yea, what about it?”
“She has Herpes, dude. We’re jungle brothers!”