Being "Faithful' is a form of Sacrifice you perform for your spouse and children

In anything worthwhile in life there is sacrifice.

Marriage is "AT BEST" a compromise.

You compromise where you are going to live.
You compromise in how you are going to live.
You sacrifice for your children.
If you are married too long before having children often you divorce because you can't handle the changes.
So, in some ways the sooner you have children the more likely you might stay married because both shocks come together (marriage and children at the same time).

When there is too much separation in time between marriage and children it often ends in divorce because the man feels rejected and left out of the marriage and doesn't know why he is still married.

Men are sort of children in this way. Being one of them I know about this.

But, being faithful is difficult for many people because maybe they were very desirable to the opposite sex or whatever and liked all the attention. Then they have to work all the time taking care of their spouse (and children maybe too) so wanting more positive attention often leads to some being unfaithful.

Often it isn't when they are feeling good about themselves but when they are most depressed about something that they are the most vulnerable.

So, hopefully most people have an active group of relationships that they have before marriage. Because the younger someone marries the more problematic this sort of thing can become.

However, the opposite can be true too when people have dated just too many other people and so the habit of dating a lot of people one kind of can't completely get over.

But, being faithful is an act not only of "Self Respect" but also of respect for your spouse and family.

Because you endanger both their sanity and future by engaging in being unfaithful. Once you are married and have children it's not just about your life and future anymore.