There are a multitude of factors. Sexism, Homophobia (dominant in men, directed towards men mostly), religion... specifically Christian (that whole sodomy bit).
If I was to get right down to the core of it, I'd say its more to do with the simple act of penetration. Lesbians, short of using implements and their digits (and other apendages) don't truly "penetrate" as for some reason penetration in sex requires a cock. It can be perceived as "purer" then male on male penetration.
It's all stupid mind you. TV and mags just emphasised these things and create a vicious cycle of people being influenced by media, while the media is also being influenced by people.