What a fascinating thread...
I'm in my 60's so have seen a bit of life and experienced a few relationships. And I'm hoping that I might just the first person who lives to 200 
I've always been heterosexual, only really sexually attracted to men. Wouldn't rule out a relationship with a woman but feel like I would need to 'do' more to keep the relationship going.
In late teens / early 20s I think I had a strong biological urge - find the right man, have kids. Despite all the possibilities for women to go it alone, I still think that biological drive is strong. There are so many threads where women are panicing about their age, and sadly even more where they compromise their ideals (e.g. marriage before babies) for the sake of having a child, and then never get the married status they are longing for.
Although there are so many options, there are still many women who seek to be settled long term with one person, and want the stability/commitment/legal protection that marriage brings, and want that person to be a partner for life.
In my 30s, with kids and 2 demanding careers and an 'old fashioned' husband who thought he was the head of the household and that he should have conjugal rights, I was convinced that living apart together was a better model. He didn't and we divorced. I think that the years of a young family are the most challenging in any relationship. Competative tiredness, resentment, unequal load in household responsibilities, financial inequality... I'm not surprised that so many realtionships break down during this phase.
However, I think all too often it lets men off the hook. From being an equal partner in a relationship with equal responsibilities for a family, they see dc EOW and the rest of the time are 'free men' to date someone unencumbered who can give them the attention they craved, and then they marry the new person and have kids, and that cycle starts all over again.
When I divorced I was so happy to be single, determined never again to live with anyone. Then I met my now dh. He was, and is, entirely responsible iin all his attitudes, our relationship is equal in all the ways that matter, after 13 years we combined most of our finances, and 2 years later we got married. 15 years on we still are. At this stage in life (he's retired, I'm doing 'hobby' work) our shared memories are the most important thing. Laughs we've had, trials we've faced together, shared ridicule at our creaking knees and lost car keys. The thought of starting again now horrifies me. I anticipate us being together until...
Well there's another stage: my very elderly FIL has dementia, he drives his previously tolerant wife up the wall. After more than 60 years together, she would prefer not to live in the same space as him 
I could imagine living in some kind of communal space with elderly couple friends (with the costs of a professional carer shared between us).
So my life/relationships are/have been conventional. My dc less so. One is a single parent, the other is in a polyamourous relationship.
My thoughts for the future are that there might be more communal living - women (and families) supporting each other, rather than everyone in their isolted box. Models for older people where they can remain part of a community even if their partner leaves or dies or is demented.
My fear is that these models might work for those who have money, flexibility, adaptability. For those who don't there may be unhappy confinement to a single ongoing 'relationship'. And I agree with others that there will be an increasing number of angry left-out men who will prey on the weak and vulnerable.
I remember seeing a seal colony where the female seals and pups were all contented and calving, there were one or 2 dominant males, and the rest of the males were just fighty, angry creatures, biting lumps out of each other.