It's true class D tower operators don't have to let you transition, but its often in their best interest to do so. In a past life as a CFI (in a -172) we gave many sight seeing and introductory flights that required a Class D transition or lengthy detour.
One busy day I was denied access. What this new controller failed to realize was this forced me to fly around the airspace and through the final aproach course of her only ILS at a busy corporate airport. Out side of the airspace I could do what ever I wanted and didn't have to respond to radio calls. Had I been cleared directly over head or through a distant part of the airspace life would have been better for all.
Now, if you're trying to land they can't deny you for being a glider but they can ask you to standby for work load reasons (which for us is a non-starter of course). Airmanship would dictate that you had a plan once on the ground to not impede traffic and get to the FBO with out tower assistance. Landing a glider is busy enough with out trying to manage all that last minute. I'd need a serious reason to head for our of our local class D relief airports.
Glider pilots don't get much experience with radio etiquette. There's a bit of a dance you must do with controllers, behave professionally and they'll let you play, stumble in blathering all over and you'll get the cold shoulder.