A pink light

Stepping inside he was bathed in a pink light that makes him weaker while making his body burn.

"I can't believe that actually worked" Metallo marveled as he pushes Superman into the inside of the vehicle where instead of seats it was a bed and a machine that spreads radiation all over the inside.

"What are you doing to me?" Superman asked as his body started to morph becoming more feminine in the process, muscles becoming smaller, Chest becoming soft and inflating into comical levels along with the ass, his manhood shrinking and being shaped into a vagina, the face slowly being contoured into a softer form with lips inflating and hair growing up to shoulder length.
"Remember our fight when that kid firestorm turned my battery into lead?" During a confrontation the fiery hero trying to change the Kryptonite into something inert it resulted in a session where the young hero kept only changing the color of the rock resulting in various effects including what happened when it turned pink that turned the man of steel into the woman of steel.
"After that incident, I spoke to Luthor and made sure to secure an artificial pink variant of your weakness, same effect less of the strength, more figure" the explanation was more to keep himself occupied as he watched the suit being torn open revealing her ample chest and ass.
When the transformation was done the now Super"Woman" laid on the bed with her costume in tatters and most of all horny.