What's degrading and what isn't is all a matter of opinion. Some women would find it degrading, while others don't. If your girlfriend does not like the idea of it and does not want to do it, that's ok.
Yes, it's totally subjective. Personall, as a female, I find it degrading to the man to spit!! Guys deal with a girl's taste when he does oral on her; she should deal with his taste. Semen does not taste that bad to me, and I find spitting anywhere at any time to be insulting, degrading and disgusting so I defiantely wouldn't do it during sex.