Because you get to watch two hot bodies go at it with each other instead of a woman and a man's sausage.
It makes more sense when you actually think about it. A straight guy is attracted to women, he is turned off by seeing another man's privates/naked body. So while two different urges battle inside the man's body, lesbian sex is twice the desired sex without the repulsive image of the one you aren't attracted to.
It makes more sense when you actually think about it. A straight guy is attracted to women, he is turned off by seeing another man's privates/naked body. So while two different urges battle inside the man's body, lesbian sex is twice the desired sex without the repulsive image of the one you aren't attracted to.