My finacee is the only guy I've ever known who can stimulate my G-spot really effectively with his fingers. Unfortunately, if he does it for more than about a quarter of an hour, I sometimes get cystitis next day. Why is this, please?

The G-spot is extremely close to the urethra, which is the urinary pipe. Therefore, over-intense G-spot stimulation can cause irritation in the pipe. Result: cystitis-like symptoms. So I think you should urge your betrothed to take it a bit easy with your G-spot in the future. A good plan is to limit stimulation to about five minutes - and to do it gently.