These men did not understand what a sin really is. The sin was not confined to the day they had the vasectomy. Every time they congratulated themselves on their cleverness, and were glad that they'd had the vasectomy, they were ratifying the sin. In a sense, they were committing it over again each time they ratified it again with their will.
Yes, I realize that they were rationalizing it away like a pack of lawyers. But, they (and I) were just doing something that every other branch of Christendom, and most other world religions find responsible, they were taking effective means of limiting their family size to what they could afford.
The idea that we were put on this earth solely to be breeding machines, in a world of over five billion people already, was one of the thousand reasons I had to leave the Catholic church. The same kind of justifications those men made for doing what they did two decades ago, I have seen in the recent pedophile priest scandal, so the attitude that was displayed by the laity extends to the clergy.