It will take a few months on HRT, and the situation will take care of itself. Your testicles will atrophy, and at a certain point, the erection feature will quit. At that point, your prostate will still be able to put out its contribution to your semen volume, but there will be little if any actual semen in it, the fluid will become much lighter, and not as much. You will have to learn how to control things, but it will come to no more wood in the pants.