I think monogamy will continue to be the default MO of relationships although divorce will become more common as life expectancy keeps increasing. I also think acceptance of other relationship models will increase but I doubt they'll become prominent among lower classes, having one partner already is a lot of work and people with little in terms of money and perspective are unlikely to be able to afford that full attention for another partner. (yes cheating is a thing, it usually also involves either a reduction in relationship activity with the cheated on or a relationship light with the affair partner)