My endocrinologist told me that while on test I will not be fertile, but he did say HCG will counter the fertility issue if I decided I wanted to have more children.
My view is better safe then sorry, if you want to have kids, don't go on TRT unless you absolutely have to. I suppose you could say don't go on TRT unless you really need it regardless, but, particularily if you want to have kids - it definitely can potentially affect fertility.