I have tried to respond to this topic, but once I type up something then there's a server issue...
20 years from now I think in most first world and western countries, the ideas of relationships and marriages will have done a type of full circle, but also evolved. I think people are tired of the government sanctioned and involvement in "marriage" and partnerships and it seems like a never ending battle that takes away from the overall idea.
I don't think the idea of lifetime partnership will ever stop, whether symbolically or just mating for life. However, I think the roles of men and women will change and the ideas and motivations for partnership will continue to evolve and change. I think we will see less marriage, I think we will see more procreation out of want, older men and women having children and waiting.
I think there will be more of a technological aspect to partnership and people actually doing things that are kind of hard to wrap our head around these days. Such as being "married" but being alone during the week, being married but actually never meeting in person, getting inseminated and everything else through sterile environments like a doctor's office. I even read a book by someone about laws made for a man or woman to marry a robot!
I think it will be an everchanging situation, all I can say is that in 20 years I hope to still be doing it and still be enjoying it for what it is worth...