@luce-anon @anathema @jackemled i agree with (1) but i think your reasoning is off for (2) and probably (3). for instance, in the direct next verse it's talking about people born of illegitimate offspring, or even the tenth generation of their children. that's definitely not voluntary

the reasoning i've usually seen cited, and which i suspect is correct, is that it's a human equivalent of sacrifice laws. it's established in leviticus that sacrifices must be of male animals "without blemish," i.e. not castrated. the sacrifice must be of the highest quality in order to pay reverence to God, and a castrated animal was degraded. these offering laws had human parallels, such as in the principle of offering one's firstborn son to God established in Exodus, though ofc these would not be human sacrifices but living sacrifices where ppl were dedicated to God in some way (e.g. with the firstborn son bit, the consecration of the Levites was thought to fulfill it iirc, which i don't totally get since Levi was the third son, but whatever). to comprise the assembly of God out of an imperfect, castrated man would have been the equivalent of offering a castrated animal to God: an offensive impurity. for that reason i do not think whether it was voluntary or not mattered. it wouldn't have been seen as a moral failing to be castrated—since this is an issue of purity, not of offence—but Leviticus is full of cases which are moral but impure

i also don't think groups self-castrating at this time is really well accounted for. radical celibacy was an issue in the early church, but that's anachronistic. eunuchs would have come about as servants of monarchs or accidentally in this period