Sforno on Leviticus 22:25/ספורנו על ויקרא כ״ב:כ״ה

1Even though gifts for the Temple treasury are accepted from pagans, no blemished animals are to be accepted from them as such. Even if the animal became blemished through a deliberate surgical procedure, such as castrating it, something which generally improves the value of the animal so castrated, it disqualifies it from use by the Temple treasury, not to mention as a sacrifice on the altar. Seeing that the gentiles consider such animals as superior, we could not apply to such gifts the criticism voiced by Maleach 1,8 which we referred to twice already. We might therefore have thought that seeing that in the donor’s eyes he presents a superior gift to G’d this would be acceptable; the Torah takes a dim view of castrating animals which were created to proliferate. The Torah describes such castrated animals as

2כי משחתם בהם מום, for their corruption is within them, (not only external). When disqualifying blemished animals for the altar the Torah did not only have in mind visible blemishes, which are after all, only external, but invisible blemishes also, the latter often being more serious.