Honestly, its possibly because she feels she does it better herself. You being there is just a distraction and maybe your not as good as you think you are at "sexual favours" and she just doesnt want to hurt your feelings. I was seeing this guy a few months ago and he claimed to be the best pussy licker, he loved it blah blah. When the deed actaully happened he was shit at it so I ended up going home and having to finish myself off, waste of time him even being there haha. Sounds harsh but some women don't climax as quickly as others and if your not doing it right then its just boring for us.
Could be completely different to what shes thinking but you need to have a blunt chat to her and don't be upset if she tells you things that you don't want to hear.

Maybe she gets off on something she knows would disgust you. Maybe she likes watching gay male porn.

Maybe she's embarrassed

My husband and I are the same it will destroy your sex life if you don’t do something about it early on

If this upsets you discuss it with her. Try and open a line of communication where you can discuss and compromise.

Maybe because of what you just said. What turns her on is what you can’t give her