The previous reference is Isaiah 56:3b-5
Biblically, eunuchs aren’t just those who have been castrated, who have been rendered physically unable to fulfil a marital duty. Jesus’ definition is as follows (Matthew 19:12):
“For there are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven”.
More on that later, but when the Lord speaks of eunuchs in Isaiah, he is talking about all who are single. And Isaiah isn’t just applicable to 7th C BC Jews: he was prophesying of the future. When he wrote of Sabbaths he didn’t mean those eunuchs who have the right theology on days of rest, but on those singles whose rest is found not in a married relationship, but in the Lord of the Sabbath. When Isaiah wrote of the covenant he wasn’t talking about singles who perfectly kept the old covenant, but those who love the Lord of the new covenant, and love others as themselves.
So Isaiah’s promises are for all Christian singles, that the Lord will give them an everlasting name within his house, the church, that is better than anything marriage can offer. It is better to have a spiritual monument in the church for being a faithful single than to have many sons and daughters.