Confused

So I was looking the girls my SO has as friends on Facebook and everything seemed fine until I came across this cover photo of a girl half naked. He commented "nice ass!!" And liked the picture I sent him a screen shot and said really. My question is........... Am I over reacting or was that really wrong of him? And yes I know it's wrong for me to be looking through his friends but he has been out of town for work for the past three weeks and he's suppose to be coming home soon and I was worried. Help please