So why else would you be okay with your wife having a girlfriend, but not a boyfriend? Why wouldn't all the things you fear about her having a boyfriend be relevant for a girlfriend? If a man makes you insecure, why wouldn't a woman?
If you want to believe that an OPP isn't about misogyny and sexism, you go ahead and stay in never-neverland. It is absolutely about men thinking that lesbian relationships bear no real threat to heterosexuality.
I believe it has less to do with believing a lesbian relationship is inferior, and more to do with how men are socialized in our society. Men are raised to be very superficial and standoffish with other men, and never become too personally close, physically or emotionally. We are also raised in a hyper-masculine society, where even the slightest hint of intimacy between two men is instantly conflated with "being gay," a label that most men fear, regardless of their true sexual orientation, due to the difficulties and stigma associated with it that have only recently become more acceptable.
Also, we as a society are raised to believe that our lovers are our property, regardless of whether you are a man or a woman. We hear it all the time on these forums, where a person describes poly as "sharing their partner." So you already have that hurdle to get over, the whole ownership mentality.
Combine that with the stand-offish nature of man and the rampant homophobia and desire to project one's masculinity, it is easy to understand why men are more likely to have a harder time with another man being involved with their partner, instead of another woman.
The final piece of the puzzle lies in the fact that the be-all and end-all sexual fantasy of most men is a threesome with two women. The likelihood of that happening is a lot better if your partner is with other women rather than other men. So there you have some selfishness, and also the potential to fulfill a lifelong dream.
Of course, there also is the one-pussy policy, but you rarely read about it compared to the one-penis policy. There seems to be an inherent ability that makes it easier for women to accept their man with another woman than men being able to accept their woman being with another man. A lot of that, I believe, has to do with how women are raised in this country and around the world. I think women are more naturally prone to being the givers in the relationship, and are also socialized to be more submissive to the sexual desires of their men.
So I really believe it's a much more complicated thing then simply believing lesbians aren't real relationships. I have read a lot about this on many forums, and have personally done a lot of introspection on it, to try and figure out why it is that I personally would prefer my wife to be with another woman than another man. I have absolutely no belief that lesbian relationships are inferior. For me, personally, there is just an ick factor that is hard to get over. And I believe that is true for a lot of men.
I have been planning on creating a post and a survey regarding the OPP, due to my constant irritation of the accusations that often gets thrown around on the topic.