As time goes on more and more people will opt of out marriage.
This is because more people will become more and more selfish and more lovers of themselves. More people will tend towards having open relationships rather than 'tying' themselves down to only one person.
The world economic situation is not likely to get any better which means more people will be 'forced' to abandon the idea of getting married at all simply because they cannot cope with the financial demands of having such relationships.
People will also find ways to pleasure themselves rather than hoping/depending that another person who is their partner will do that for them.
It doesn't get any better because as religious institutions crash and the world becomes more and more secular, people will become more open-minded and pursue their happiness outside the norms set or dictated by religious bodies. One man, one wife will be the first law to be struck out!
Therefore, the prospects in the coming 50 to 150 years is so so bleak!