Is it true that men with diabetes can not have children?

No. People with diabetes are in various stages. I suppose if you did not take care of yourself and ended up with lots of complications there might be a problem.

Please do not have unprotected fun, you might end up being a father when you did not want to be.

Because diabetes can affect the vascular system, men with diabetes may eventually develop some problems with impotence. However, even that can be dealt with via various medications/devices. I know of no reason that men who are diabetic cannot father children.