A diabetic patient either type I or type II is on regular medication and has kept his glucose levels in normal condition will he become infertile. And if he becomes infertile can we say that it is due to diabetes.
Diabetic conditions alters sex steroids and affects gonadal axis (in animals and humans) in that way it leads to sexual disabilities (erectile dysfunction etc.,) and also organ/cellular (gonads) damage due to glucolipotoxicity. Further, one may become subfertile status not exactly infertile.
What you said holds true, but a person with diabetes is on proper treatment maintaining his insulin levels. Does he can become infertile rather subfertile. And you mean to say that that subfertility complications might be due to glucolipotoxicity and so we can presume that its due to diabetes.
Optimal glycaemic control is crucial to sucessful management of diabetes mellitus, however, we should not forget that diabetes is a chronic disease that runs its natural course; good control of blood sugar only slows it down. Therefore,it is possible that reduced fertility can still complicate the disease despite an apparently good glycaemic control.