HA! I see it the other way. Not that you're a wuss for accepting your girlfriend for who she is - but that you are a real man. Those tough guys that don't tolerate anything often find themselves alone. Just sayin'. Your internal monologue is lying to you on this issue.
Now, that doesn't mean *you* may not ultimately be happier in a mono relationship. If that's true, well than that's who you are and this experience will help you figure it out. But be sure if you do choose monogamy, it is because that's what makes you happy; not because you're a wuss if you're not mono or out of some paranoia that poly is a secret female agenda. (Thanks for the laugh btw.)