There are many factors but the decline of religion imo is one of them.
Marriage and family has traditionally been a religious institution, or at least one sanctioned by the church or other religion. People are less religious now and this has gone hand in hand with breakdown of the family structure and divorce.
You can call this bull but if the average guy isn't religious, what's stopping him/her having their cake and eating it IF they think they can? That's the general attitude on the street nowadays: if you can get away with doing X, why not? In fact, you'd be labelled naive if you didn't and probably made fun of. People do do good because they are inclined to, so I'm not saying irreligious people aren't capable of doing good, but if you look at the masses in general, it's one less major deterrent and lack of moral structure. People are generally more selfish today as a result.
Also it's too easy to get married and it's too easy to get divorced.