Dr couldn't tell sex. Wants to wait 2 1/2 more months till next ultrasound!

I'm a little over 18 weeks and dr Scheduled ultrasound to tell us the gender. Baby wouldn't really cooperate so he wasn't able to tell us yet. Which is fine except he then said he won't do another ultrasound until I'm 28 weeks! After we got home my husband and I talked about and are now quite upset that we will have to wait until almost 30 weeks to find out the sex of our baby. I'm frustrated bc I want to start planning my baby shower, do my registry and paint and decorate the nursery. Is it wrong or rude of me to tell my dr we don't want to wait that long before our next ultrasound? I've had a great experience with this dr so far and I don't want things to get awkward between us.