I began this thread on the assumption that we'll soon be living a lot longer. If climate change and overpopulation don't wreck everything, and we find a way through, I'm pretty sure we'll find ways to slow physical ageing. I wouldn't be at all surprised if we're able to extend the lifespan to 120 or 150 – and in good health. 'Cycles' is a good word. Maybe, if the lifespan does increase, we'll evolve a less romantic attitude to relationships. I mean, go into them with the idea that they tend to follow cycles, as you say, and that we must be realistic. So, for example, we'll agree that "OK, we're signing up to produce and raise children together. We'll stick it out until they leave home, and then see how we feel. If we're happy, we'll renew the contract, if not, we'll try and part as friends."
If I had to predict what will happen over the coming decades, I'd say:
First, the number of women who divorce and vow never to re-marry will increase.
Second, the number of people (especially women) who choose never to form a serious relationship, but are happy as long-term single, will also increase.
Third, there will be a huge increase in misogyny and sexual violence. Women no longer need men (they may like them, love them, care about them, enjoy being with them, etc, but they don't need them). They are becoming more choosy, and less desperate. As a consequence, the worst sorts of men find it harder and harder to find anyone. Women won't put up with their shit anymore, and it makes them furious.
Fourth, marriage will become less common, but so will divorce, because the people who marry will tend to be the ones who really want to. In other words, there will be fewer people in long-term relationships, but the ones who are will tend to be happy and fulfilled.
Fifth, long-term/committed relationships will become just one of many lifestyle choices, rather than the norm.
Sixth, friendship will become more and more important. People will devote as much care and time to their friendships as they used to devote to their marriage. There will even be 'friendships counsellors'.
Seventh, we'll continue to experiment with different ways of living – multiple partners, sexless marriages, open marriages, committed but not living together, buying houses with friends instead of partners, and so on.
Still, humans are terrible at predicting the future, so that's probably all balls.