When Jesus was on this earth, one of the ways He taught people was through parables. He used stories with characters that people could understand and circumstances that they could relate to in order to teach a greater lesson. For instance, someone familiar with farm life would understand what it meant to get ready for the harvest. This same teaching principle holds true with marriage.
Marriage has its earthly benefits, as was evident when God made Eve to be a helper for Adam. This partnership is also a way to teach the importance of our relationship with God. When we become Christians we have made a promise to God that we are going to faithfully serve Him all of our days. When we get married, we vow to our spouse that we are going to stay faithful to them all of our days. We did not promise God that we will be faithful except when times get hard, and the same goes with our spouse. If we are unfaithful toward our spouse, it is the same as being unfaithful toward Jehovah. God sees this as being so serious that He made adultery the only stipulation for a person to be able to get a divorce and be allowed to remarry someone else afterwards (Mt 19:1-9).
So when we ask why marriage is so important and why adultery is such a big deal, we should also ask why truth and honesty are important as well. Throughout the Scriptures we read of condemnation for those who speak lies and are false witnesses. The reason it is such a big deal is because Jehovah is an honest God (I Sam 12:24), and Satan is the Father of lies (Jn 8:44). If we break our promise to our spouse then we are breaking it to God, and we are following in the footsteps of Satan. Yes, it is that serious.