While this discord leads from a discussion on divorce, the disciples weren't saying it would be better to "fool around". They were astonished that Jesus would not be sympathetic to divorcing for "any reason"
Matt 18:3 "Some Pharisees came to Jesus testing Him and asking, "Is it lawful for a man to divorce his wife for any reason at all?"
The disciples are especially astonished at Jesus' answer since Moses permitted it and He seems to contradict what Moses said.
He said to them, "Because of your hardness of heart Moses permitted you to divorce you wives; but from the beginning it has not been this way.
Jesus said it is because of man's failings that we fail in many areas-marriage included. As you probably are aware, not divorcing is supported in other scriptures. (I Co 7:12)
For the disciples, as with many of us, this answer is clearly a problem and the disciples come to a natural conclusion, that it is better not to marry than to make a marriage vow to a spouse and God and break it. Jesus knew that he would not be able to keep a vow to a spouse and would not dishonor God by breaking it.