A random thought...

I just realised that the phrase "pain in the ass" alludes to the homophobia of the English language. Why is it pain in the ass and not pain in a certain metaphorical feline creature? After all, incursions into both probably result in similar pain - assuming of course that the incursions are a first for both subjects; otherwise, there might not be pain, but I'm digressing.

So why is it that pain only exists in the encroachment of the former and not of the latter? Of course, one can then argue against it with the example of "pain in the neck". However intrusions to the neck are present in both the conservative and alternative lifestyle groups, so that example doesn't completely negate the homophobic nuances of "pain in the ass".

Would any budding etymologist care to enlighten me on this?