Picture of my wife and co-worker, please help?

Until early last week, my wife had worked with a male co-worker of hers for 5 years. He found a job out of state and my wife along with two other girls who worked with him decided to give him a going away party on his last day. Since he is a rather large man, one of the other girls thought it would be fun to have for each of them to have a picture taken sitting on his lap wearing something cute. My wife told the girls she wanted to ask me first to see if it was ok since she was the only one among them who was married. Since I knew they were good friends and I trust my wife, I told her that it was fine. My wife decided to wear a white blouse tucked into a red mini skirt. She said that tucking in the blouse would look professional while it would also look cute with a red mini. She looked incredible. Anyway, the picture turned out really cute. In the picture, she is sitting on his lap at an angle. He has right arm around her back with his right hand on her hip. His left arm is around the front of her with his left hand on her tummy.

However, whenever I see the picture, I get an erection and would like to know why I like this picture so much?

Also, please address why he would put his hand on her tummy in the picture. Thanks and please be serious and detailed if possible.