After answering the Pharisees the disciples questioned Jesus further. His answer was "'Whoever divorces his wife and marries another woman commits adultery against her; and if she herself divorces her husband and marries another man, she is committing adultery'" (Mk 10:11-12). Their reply was "'If the relationship of the man with his wife is like this, it is better not to marry'" (Mt 19:10). Jesus' use of the word "adultery" appeals to what he knew it would communicate to his audience. The disciples got the message but they overreacted. Jesus used figures of speech consistently in his teaching. When Jesus said "'Our friend Lazarus has fallen asleep'" (Jn 11:11) the disciples responded "'Lord, if he has fallen asleep, he will recover'" (:12). Jesus then clarified "'Lazarus is dead'" (:14). He also had to quash their reaction to his statement on divorce by saying it is "'given'" (Mt 19:11) to most to be married. He didn't respond by warning that when you do marry, make sure that it is for life, because you will go to hell if you divorce your wife. If that had been his meaning then committing adultery might refer to sexual acts of it. Jesus is not being so picayune that he is isolating a sexual act in this scenereo and magnifying it to make a point, but is figuratively using the word to show how serious it all is.
Consider what the disciples would conclude after they had a chance to contemplate what Jesus said. They would realize that it was wrong to be divorcing for superfluous reasons and that it would be equivalent to committing adultery. Their statement of not marrying at all would be interpreted as being the only practical way of avoiding the consequences. Does the context say that a divorce in the reestablished scheme of things would not be honored and that in a succeeding marriage they would be "living in adultery" because the original would still be in effect? Some would say that it is impossible to break a marriage covenant because that is why it is said that adultery occurs as a consequence of a divorce and remarriage since they must be still married for adultery to occur by definition. It doesn't appear to say that, or even imply that each occurrence of sexual relations in that context would be an act of adultery. This whole aspect is acceptable to someone if you are "'able to accept this'" (Mt 19:12). It connotes a responsible approach by Christians making Godly decisions in individual situations which is different than trying to keep laws which are no longer required by the New Covenant.