I suspect female bisexuality is more acceptable because (1) it’s less threatening (and may even be exciting to) males. Many men feel threatened by bisexual or gay men. (both of these notions are other questions here). (2) Women often seem less panicked than men by the concept of other women finding them attractive AND (3) female friends are more likely to express feeling of friendship physically. I sometimes wonder if both of these realities aren’t partially connected to the fact that we all begin life on intimate terms with a female body, and it therefore seems more natural for everyone to respond physically to women (sexually or otherwise). (4) Male-male sex can come with something of a physical price, making it more problematic for men than male-female sex. Female-female sex actually presents fewer complications that male-female sex for females (ie no risk of pregnancy). This means that it is physically easier, at least, for women to be bisexual. (5) There is also a theory that women have greater ‘erotic plasticity’ than men, and more inclined to modulate their sexual inclinations with the times. I do not know how accurate this theory is, but there’s no question female bisexuality has a certain trendiness in some circles.