It's like getting turn on if you see a man massage a woman's feet. It's not some inborn thing that's the product of genes - it's a fetish. And despite our sex-phobic culture, there is *nothing* wrong with having a fetish. Fetishes are amazing. They lead to great and unconventional sex. Particular fetishes can be problematic (e.g. from a hygiene or consent POV) but that's not a problem with fetishes generally.
Getting turned on by two women is not biology. It's - as other posters point out - a result of a cultural construct that lesbian women are somehow always supposed to be sexually available for the venerated "ideal" threesome.
I am thinking about me right now, when I read this sentence, and I can't help but smile.
I never in my life dreamed of a threesome, I don't ever want to experience that in real life, even If I could find it fun in a random movie, I don't ever want several partners in my bed, only the chosen one. It never will be my ideal, never dreamed about it... And yet I could appreciate two chicks kissing each other, having sex, in a movie, a video game or whatever you'd like. I also never dreamed of lesbians available to me. I'm not into someone who doesn't find any appeal in my gender, in who I am, as a human being. I don't hold any sexual interest toward someone if I learn that she is lesbian and never will. I will respect what she is, and what I am. The girl could be the most beautiful girl ever, that wouldn't change anything in my mind.
And yet, you are telling me that because I could appreciate, as a straight guy, who can't be concerned because of my gender ( while I didn't watch many lesbian relationships or sex, I'm not a noob ) lesbian sex, I dream that they are sexually available to me, a threesome is my ideal.
I'd also like to know how culture influenced me.
I just want to be sure I didn't fail to understand what you meant.
I'm also curious about what you think about all those women who are fond of yaoi mangas, very popular thanks to them. Do they want a threesome with gay men, is it their ideal that all men are sexually available to them ?