Although Pompey started the game well, with Kanu testing Albion 'keeper Peter Brezovan early on, the day would go drastically downhill after Liam Lawrence was about to take a free kick.
In a moment of madness, Dave Kitson reacted to Adam El-Abd's "physicality" in the box by lashing out and earning him a straight red card, meaning an instant three match suspension.
Things got worse when Chris Wood gave the home side the lead. It was difficult to see how the goal was scored from the other end of the awful Withdean Stadium, but yet again the Pompey defence weren't tight enough in the box and allowed Mauricio Tarrico's shot to fall to Wood so he could score.
The Seagulls continued to dominate the first half as the ten men of Pompey struggled to keep up with them. Ashley Barnes hit the bar in a period which saw many Pompey players booked for dissent after Nugent also received a yellow card for a late challenge.
Brighton continued to pour forward and would have doubled their lead if it wasn't for a couple of a couple of great saves from Ashdown who was the only Pompey player to emerge from this wreckage of a football match with any credit.
However, in the final minute of first half stoppage time, Brighton were awarded a penalty when Ibrahima Sonko needlessly handled the ball in the box.
The view from the Pompey end of the stadium may have been poor but it was clear enough to see Sonko's silly mistake and Barnes was more than happy to accept the gift and give his team a two goal lead that killed Pompey there and then.
The second half was equally depressing as Gus Poyet's men continued to exert total control of the match. The only real chance for Pompey came when Hermann Hreidarsson, bemusingly on at half time for Kanu, headed Sonko's cross wide when he should have scored.
Things went from terrible to atrocious when Liam Lawrence pulled up with a hamstring injury and had to be replaced by Tom Kilbey.
The youngster pulled a goal back in stoppage time when he drilled the ball through a crowd of Brighton players to give the visiting fans hope.
But that hope was immediately extinguished when the opposition went straight down the other end and scored when Francisco Sandanza finished at the second attempt after Barnes was allowed to run down the wing unchallenged.
This Pompey performance was nothing short of shameful. No quality, no discipline and no effort as we were outplayed by a team a division below us. The early red card didn't help our cause but to get seven yellows in the game proves that the players lost their cool when they needed it most.
This is going to be an extremely testing time for Steve Cotterill now. With his squad seriously depleted he must now make do without Kitson for three games and who knows how long Lawrence will be out for?
He also needs to answer some questions about his own methods as fans are now starting to question his tactical decisions. Bringing Kanu off was baffling as he was our only creative player on the pitch but to replace him with Hermann and push Dickinson into midfield? I'm not sure how that was supposed to work.
The manager shouldn't shoulder all the blame though. Our defenders were woeful yet again as Dickinson, Mokoena and Sonko made mistakes and were shown up for their lack of pace whilst Ward lost the ball too many times.
They weren't helped by a midfield that didn't seem to know what it was doing and didn't seem to be up for the battle, but at least Nugent put in a shift unlike Utaka who has returned to his old self.
Perhaps Cotterill needs to go back to basics and make the team perform defensive drills in training in preparation for a long, hard slog at Forest next Saturday.
We are far too easy to beat right now and that needs to change before we get dragged into yet another relegation battle.
Pompey Player Ratings
Jamie Ashdown: 7 - A couple of great saves to keep the score down.
Joel Ward: 5 - Lost the ball too many times.
Aaron Mokoena: 4 - Very sluggish.
Ibrahima Sonko: 3 - Absolutely woeful.
Carl Dickinson: 4 - Poor yet again.
David Nugent: 6 - Worked hard.
Liam Lawrence: 5 - Didn't see much of the ball.
Hayden Mullins: 4 - Looked off the pace.
John Utaka: 4 - Ineffective.
Kanu: 6 - Provided some creativity.
Dave Kitson: N/A - Stupid sending off.
Substitutes
Darryl Flahavan
Hermann Hreidarsson (Kanu 46): 5 - Missed a great chance.
Tom Kilbey (Lawrence 79): N/A - Grabbed a poacher's goal.
Nadir Ciftci (Nugent 87): N/A - Barely touched the ball.