Caught My Student Jerking Off In The Library Today

I walked into the library to find one of my students, a guy in his early twenties, with a boner and his hand around it. He looked shocked when he saw me. I leaned over him, whispered for him to keep going. Pulled out my phone, snapped a quick pic. His eyes widened, then dropped as he kept jerking off faster. I slipped the pic into my bag before he could protest, left with a wink. Is this normal? Should I blackmail him or just laugh it off?