How so?
To summarize the principles of moderate realism (Aristotelian/Thomist moral philosophy) as concisely as possible:
all human faculties are naturally ordered to an end.
The end of the generative faculty is procreation.
The destruction of a faculty's ability to achieve its naturally ordered end, when that destruction is not ordered to a higher end, is violative of that faculty and constitutes a mutilation of the person.