Marega, 28, walked off the field in disgust after being subjected to a torrent of monkey chants after scoring the winner in a 2-1 victory over his former club on Sunday.
"With all the insults I received, with the monkey chants targeted at me, I couldn't stay on the pitch," Marega told French radio station RMC.
"You can't stay like that and keep playing a match when people mock your skin colour."
In the 71st minute, Marega signalled to the bench he was leaving the pitch in protest even though teammates, Porto coach Sergio Conceiçao and Vitoria players tried to get him to stay on the pitch.