The main reason, of course, is that it is a defence mechanism against men who persist in believing that lesbian sex is second-rate [or that we can't have sex at all] It is the easy way out, to bash back at them, to justify your own choices. It is easy to point men as being insensitive clods, to highlight that two girls would be more sensitive to each other, and that two girls would know how to make better love. In essence, we claim that lesbian relationships are BETTER, and that straight relationships are duds.
Secondly, it is a manifestation of our insecurities. Decades of bad media portrayal and societal brain-washing have given us a certain inferiority complex, some amount of internalised homophobia which is difficult to get rid of. And of course, there is the perpetual fear of your girlfriend 'leaving you for a man', or 'turning straight'. This comes not from an inherent flaw in lesbian relationships, but our own insecurities, propagated by a patriarchal society. We virulently lash out against anything that might threaten our fragile [or so-perceived] world, held together by spit and ducktape against the homophobic world out there.
In essence, we behave like homophobes, who blast anything that threatens their nice little straight world, and relegate gay relationships as being second-class.
My question is: does it matter? If your girlfriend left you for a man, the reason probably isn't that lesbian relationships are lacking, it is that YOUR relationship was lacking in something, or that she simply wasn't a lesbian in the first place, in which case it is not you, but her. And is it much different from her leaving you for another woman? We criticise men for being angry if their girlfriend leaves them for another woman look in the mirror, lady, you are doing the same. But our insecurities take over our rational thinking, and we bring out the misoandric whips.
Heterophobia, biphobia or misoandry isn't the way to deal with homophobia or specifically lesbianophobia. It only reinforces the stereotype that we are all man-hating bull dykes. Why give the homophobes that satisfaction?