As the pattern of thought goes, if a woman meets a gay man, then that's it. Since he's not going to pursue her, she may as well give up. But if a woman rejects a man on the basis that she's lesbian, then "clearly" either she's lying, or he's not trying hard enough. After all, everyone knows there are no real lesbians, there are only women who haven't met the right man yet. The existence of the "lesbian except for you" trope in male-oriented media doesn't help, either.
To be entirely accurate, however, it is in fact common for women to claim they are gay (or have a boyfriend, more commonly) when they aren't (or don't) as a rejection tactic, but it is not at all common for the reverse to happen.