Matthew 19:10

His disciples say unto him, If the case of the man be so with his wife, it is not good to marry.

The verse records the disciples concluding that if a man's obligation to his wife is as Jesus taught, it is not good to marry. The strict teaching they were reacting to was that divorce is allowed on only one ground, fornication, and not for every cause, and that marrying another after an unlawful divorce is adultery. They reasoned that if marriage could not be ended freely or on easy terms, singleness seemed preferable, and they found the instruction difficult, with the following words showing that some were too spiritually blind to receive it.

In reply Jesus explained that not everyone can accept a single life: some remain unmarried because of circumstances of birth or constitution, some because of mutilation by bond-masters, and some by choice for the kingdom of heaven. Those who remain unmarried are different, not better than others, for marriage rests on the original order established in Eden by Christ, which should never have been departed from and remains necessary despite sin.

As part of that Eden-based teaching, the verse belongs to instruction that rejects polygamy as evil and wrong and bringing great unhappiness into families. In giving this teaching on marriage Christ was expounding his Father's law, not revising it or establishing a new law. The wider chapter teaches that keeping the commandments alone is not enough to follow rightly; as the rich young man had to sell his possessions and give to the poor because his heart was attached to wealth, so a person must surrender whatever holds the heart back.