My pride plays a part in this twisted philosophy. If my boyfriend breaks up with me because he is gay, I figure, it's not my fault. His homosexuality is to blame, and not any shortcomings on my part.
On the other hand, if I find out that he is cheating on me with a girl, then a thousand insecurities would attack me: What does she have that I don't? Do you find her prettier than me? Funnier? More clever? More interesting?
If a boyfriend cheats on me with a guy, then I know that my only shortcoming is not having a penis. And that is not my fault.