The lesbian sex that is shown off in porn for the run of the mill straight guys is a bit... showy. All of them (from videos to magazines) are so femme. Just bring some porno actresses, and have them do to each other as if they're doing a man. The women are there to be exhibitionists for what straight guys think is sex between women. It's the same rationale of associating gay men with anal sex and lesbians with cunnilingus. Whatever they can do with each other, breeder folks like myself will never know (unless any of us are bisexual).

Joe Schmo with the VCR (or DVD player) remote does not feel threatened by seeing another well-hung guy onscreen. It's like the "lesbians" are screwing each other just for him.

Or I can say that guys will think "Oh yeah, they'll screw everything that moves" when they see a porno actress or any other women who had sex.