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I've always loved seeing two girls together, but hated two boys. I finally realized why some years ago:
I would love to have a sexual interaction with two gals, but I know that if those two were only interested in ME, they would be sorely disappointed. There is absolutely no way I could satisfy any two women I've ever known during a sexual encounter, no matter how much I might like to. They would have to be taking care of themselves and/or each other to fully enjoy themselves, and I hate to leave anyone hanging during sexual activity. If the two gals were interested in each other to begin with, I wouldn't have the concern about me being responsible for the ultimate fun.
So, with Lesbian Porn, I get excited about it because I can often see myself in the picture with them. Likewise, I can see myself in the picture with a single woman or a M-F couple. It is the fantasy of ME being in the picture that excites me about any of these porn scenes, not the voyeurism aspect of watching them get themselves off. For the same reason, F-M-F porn turns me on, but M-F-M tends to turn me off.