I'm beginning to see a parallel between this issue and the good old "I don't have a problem with X being gay, except that up until now he/she has always been portrayed as straight - making him/her gay is not being true to the character."

While this sounds reasonable on the surface, it ignores the fact that the default orientation for characters has always been straight - just like the default sexuality for "good" female characters has always been committed and monogamous. You need to actively declare a character gay; there is no need to declare them straight, unless you state otherwise that's just assumed. The burden is on the writer to declare that a heroine might enjoy casual sex - because otherwise it's conventionally assumed that she doesn't.

And if a writer does make that declaration, people will be unhappy with it because she's never been written as that "type" before.