Love this question, and waiting to see the answers. I know I sure couldn't answer it with any accuracy.
Maybe some of it has to do with the fact that almost every porn movie has some sort of girl/girl action in it, and they are so widely viewed it has made it more normal for girls. I know there are gay guy/guy movies available, but unless you are into that life seldom would you see the movie, whereas the standard porn movies are watched by an enormous amout of people.
i agree that society is more accepting of a female experimenting with bisexuality. while I really have no idea why, my guess is that it has something to do with a number of factors, including (i) the generally accepted male/female role in sexual relationships - that is, men are typically considered to be the hunter and women the hunted and more vulnerable, (ii) the general view that the body of a women is much more beautiful than that of a man and (iii) the stigma (whether right or wrong) associated with being a gay male - which is what many people would believe if a man were to experiment with other men. personally, I have experimented with other men and I love it. I am bi and see no problem with men experimenting with other men. the extent of my experimentation does not involve kissing though. it is not so much the man I am interested in as what the man has.