No, it is NOT unfair or sexist at all, but thanks for letting us know how UNAWARE of the real world = you are.
The surgeries are not the same. ANESTHESIA costs money – the longer the surgery takes, the more anesthesia is used. Spays are invasive and complicated, The incisions are larger, therefore more time consuming to sew up and the vet’s time (if measured as most workers) – by the minute or by the hour, will also COST MORE on a spay.
Now would you also like to tell a plumber the next time you have broken pipes, that regardless of how many or how much pipe LENGTH he has to replace – the cost should be the same? He will laugh, and walk off the job.