"If it had been Plastic Amy settling down to quietly guard Pandorica Rory for two thousand years, would we call it heroically sacrificial love or pathetically needy inability to exist without her man?"
I think an important consideration is that word "guard". Rory isn't just sitting around waiting for his sleeping princess to wake up; he's doing something active and useful. If Plastic Amy did the same, it would be all right - the question is, would she be allowed to? Cultural assumptions might act against the audience accepting, or the writers expecting the audience to accept, the girl as a guard and protector, even if she is a plastic girl with her own ray gun. There might be a temptation to make her do something more 'womanly', like nursing her sleeping prince - which would be ironic, because (bringing us back to your general point about Mr Pond and gender politics) if there's nursing to be done, the person who'll be doing it in the current TARDIS crew is Rory.
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I think an important consideration is that word "guard". Rory isn't just sitting around waiting for his sleeping princess to wake up; he's doing something active and useful. If Plastic Amy did the same, it would be all right - the question is, would she be allowed to? Cultural assumptions might act against the audience accepting, or the writers expecting the audience to accept, the girl as a guard and protector, even if she is a plastic girl with her own ray gun. There might be a temptation to make her do something more 'womanly', like nursing her sleeping prince - which would be ironic, because (bringing us back to your general point about Mr Pond and gender politics) if there's nursing to be done, the person who'll be doing it in the current TARDIS crew is Rory.