I think its because most (almost all) bi girls want to have a boyfriend and girlfriend at the same time and expect the girl to understand its because of society. I dont think this is fair to the girl bcz most times they come second to the man. That's why it is easier for a bi to date a bi than a lesbian, unless the bi girl is ready to be with the les girl only, or be with the boyfriend only. This is just my point of view and from the bi girls i hv met.