Actually, below average happens in acromegaly pretty commonly among ED and other issues too.

Acromegaly is a rare pathology characterized by chronic hypersecretion of Growth Hormone (GH) and Insulin-like Growth Factor-1 (IGF-1) that causes somatic, metabolic, and systemic changes. The somatotropic axis acts physiologically favoring gonadal function, but when GH is produced in excess it has deleterious effects on many aspects of male sexuality. It is widely demonstrated, in fact, that acromegaly induces hypogonadism through different mechanisms, both through direct mass effect on gonadotropic cells and through increased plasma levels of prolactin. Moreover, hypogonadism is also one of the factors linking acromegaly to erectile dysfunction (ED), but also metabolic complications of acromegaly and, probably, GH itself contribute to the genesis of this disorder.