It's very simple. There are no men in the picture. It's pure abstract sex. If a man sees another man having sex with a woman, he thinks "who the hell does that guy think he is, having sex with that woman I should be having sex with? It's an outrage!" There is no such barrier when watching hot girl-on-girl action.
Women think totally differently. They see a hot guy with a woman and they'll ask a man - "do you think she's pretty?" Men couldn't care less what other men look like.
Male fantasy. :o) You're always hoping that you'll be asked to join in. That's why the response.
"If a man sees another man having sex with a woman, he thinks "who the hell does that guy think he is, having sex with that woman I should be having sex with? It's an outrage!" "
Which explains, I suppose, why men don't watch explicit porn, then. Oh, wait....