Mat 19:8 He said to them, “Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so.
9 And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery.”
10 His disciples said to Him, “If such is the case of the man with his wife, it is better not to marry.”
11 But He said to them, “All cannot accept this saying, but only those to whom it has been given:
12 For there are eunuchs who were born thus from their mother’s womb, and there are eunuchs who were made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the kingdom of heaven’s sake. He who is able to accept it, let him accept it.”

Verse 11 renders the previous statements less than absolute "all cannot accept this saying, but only those to whom it has been given"
He doesn't define non-acceptance as a sin but rather as a lack of having been given the gift to accept it. "he who is able to accept it, let him accept it". The implicit is that those who aren't able to accept it shouldn't.

Who is a "eunuch who has made himself a eunuch for the kingdom of heaven's sake" ? Note that the New Testament doesn't portray eunuchs in a negative way, the account from Acts 8 where Philip preaches the gospel to a high ranking eunuch demonstrates that.

1 Cor 7:5 Do not deprive one another except with consent for a time, that you may give yourselves to fasting and prayer; and come together again so that Satan does not tempt you because of your lack of self-control.
6 But I say this as a concession, not as a commandment.
7 For I wish that all men were even as I myself. But each one has his own gift from God, one in this manner and another in that.
8 But I say to the unmarried and to the widows: It is good for them if they remain even as I am;
9 but if they cannot exercise self-control, let them marry. For it is better to marry than to burn with passion.

The implicit is that Paul had made himself a eunuch for the kingdom of heaven's sake, meaning he had decided not to marry and that a better ability to live like that had been given to him by God as a gift. Jesus ties the acceptance of his call not to divorce then remarry to being given this gift. Everyone does not have this gift, as Paul himself points out elsewhere :

1 Cor 9:4 Do we not have a right to eat and drink?
5 Do we not have a right to take along a believing wife, even as the rest of the apostles and the brothers of the Lord and Cephas (Peter) ?