Adam Johnson has damaged the reputation of football, according to Professional Footballers' Association chief executive Gordon Taylor, who admitted the winger was unlikely to play again.
The former Sunderland and England player was jailed for six years at Bradford Crown Court on Thursday for engaging in sexual activity with a 15-year-old fan.
Taylor said in an interview with BBC Sport that his organisation took the issue of child protection "very seriously," but accepted the Johnson case was an "example of something going badly wrong".
"It's anything but good. It's very damaging for the reputation of the game," Taylor said.
He added: "Just to reassure you and everybody out there, this is being taken very seriously.
"It's not as though we don't know the law of the land and we don't expect footballers to be immune from it. They're considered to be role models and from that point of view we need to reiterate that and to work even harder on these child protection and safeguarding issues."
Taylor said players had to be "very, very mindful" not to cross boundaries with young female fans, who presented a "danger zone."
He said: "Of course there will be times when there are danger zones. With young women, particularly girls, then that's even more dangerous. So they need to be very, very mindful and there's a line beyond which they just do not cross."
Asked if he thought Johnson would play football again, he said: "At this moment in time that looks particularly remote."
Taylor said the PFA had yet to decide whether it would continue to offer support to Johnson.
"His position with regard to rehabilitation and his incarceration will be discussed by the management committee of the PFA," he said.
The NSPCC on Thursday wrote an open letter to Football Association chairman Greg Dyke saying it is worried football clubs are not taking the issue of child protection as seriously as they should.
The letter, signed by NSPCC chief executive Peter Wanless, said the organisation was worried football had a "cultural problem."
It said: "This is not only about one rogue player that behaved badly, but a club that seemingly did not have child protection priorities embedded into their culture.
"It was not equipped to handle these allegations and seemingly did not deal with them appropriately, or indeed seriously.
"Furthermore, it is concerning if this is being forgotten by a club at the top level of football."
The NSPCC said it "would like to work with the FA to hammer home the message that this kind of behaviour should not be tolerated at any level and certainly not brushed under the carpet."
Johnson was suspended by Sunderland after his arrest last March, but returned to training and continued to play for the club until his trial last month.
He pleaded guilty to one charge of sexual activity with a child and one charge of grooming. He was found guilty of another charge of sexual activity with a child, and cleared of a third.
He was sacked by Sunderland straight after his guilty pleas.
Taylor reiterated he was "as shocked as anybody" at Johnson's guilty pleas.
"That was quite a surprise, to say the least," he said.
Taylor said the PFA was doing what it could to educate players, but admitted: "This shows we can't be complacent at all.
"We don't have any laurels to rest on.
"I do know my members and this is an example of something going badly wrong."