Perhaps you're putting her on a pedestal because you find her 'beautiful' and therefore too 'good' to punt with? And maybe a sluttier-looking girl does it for you?
Or perhaps although she looked great, she's not actually that sexy?
I don't think it is that. I've been lucky in life in having experienced both very beautiful WGs and Civvies - without problems associated with getting turned on (or not, as the case may be).
I've been thinking about it a lot since I posted the question. Upon reflection, and reading the above comments, I think it might be that even though the WG in question looks amazing, her service just isn't as good as other WGs who I am not as attracted to. I think her service isn't as genuine and doesn't feel as natural.
I think the best thing in the world is when you have a session with a WG and it doesn't feel like a transaction for services, but more like the time you would spend with a a civvie GF (hence, why I go for GFE). This WG, although pleasant enough, just doesn't seem as 'into it' as other WGs and perhaps I am subconsciously picking up on that and it is turning me off