Rimjob and lack of reciprocity

Hi, during foreplay, when I go down on my gf, she would often (95% of the time) put a leg up to show her butthole and it always meant rim me you know....

Monday I asked if she could do the same to me and said no, I understand the refusal and won't ask again but I feel kind sad we don't operate on the same level when it comes to give pleasure

How can I explain that to her?