City lost skipper Carlos Tevez to injury shortly after half-time before Song doubled Arsenal's lead with a curling strike and substitute Bendtner made it three with an assured finish late on.
"It looked to me that it was a sending off from the bench and the referee had no option," said Wenger, whose side had Denilson, Fabregas, Song and Johan Djourou booked in a feisty first half. "After the sending off it became electric and we got some yellow cards quickly. It was important that we did not get a second yellow.
"I asked the players at half-time to focus on playing in a positive way and to try to score a second goal, to focus on playing football and not just defending our advantage."
City had a number of good opportunities but the much-maligned Lukasz Fabianski had a fine game in the Arsenal goal, producing two particularly impressive saves from David Silva. Wenger explained: "From game to game, he shows what we see in training. Sometimes in our job you have to be steady and fight against opinions.
City still 'one of the best' - Mancini
"It is easier when you know a player has the talent and I believe he has the talent."
When asked if Fabianski could retain his place when Manuel Almunia is fit again, Wenger added: "You always want the freedom to change when needed but looking at his performances at the moment you cannot fault him at all."
City boss Roberto Mancini felt the sending off was harsh and his team would have won otherwise.
The defeat resulted in City dropping from second to fourth in the table, with both Arsenal and Manchester United passing them. Mancini said: "I think against Arsenal it is difficult to play 11 versus 11.
"Ten against 11 is worse but it is my opinion that we if played 11 against 11 we would have won this game.
"I am very proud of my players because I think they played very well. They tried always to score, always to defend very well. I think they showed we are one of the best teams in the Premier League."
On Boyata's red card, Mancini stated: "I do not agree with the referee. I thought the rule is always the last man, if there is a clear opportunity to score a goal, it is a red card.
"But I think when Chamakh touched the ball, the ball was probably going out or on the way to the keeper."
Mancini played down fears that Tevez had suffered a serious injury and added: "He took a knock on his muscle in the first half in a tackle but it is not serious."
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