I think it all depends on the couple, some have open relationships and some have closed relationships, I think it depends on what the couple want and will exept.
If you are refering to being in a 1-1 relationship as being stable, the coupling is stable, but the relationship may not be stable, one may have bad finances, but they may be in a stable relationship....... while another couple may have an open relation and have super good finances, it all depends on the situation. The people and what's considered stable to them.
I think stability lies today in what one wants and what's acceptable to them, like my divorced parents, I'd always wanted them to stay together, they'd be stable for the family but not for each other, they would be unhappy.
If you are as I said in reference to 2 people in a committed relationship considered stable, yeah to most it would be.......... but in today's society it's really hard for that to be the true utopia, it's everyone's ideal..... but temptation's always around the corner, so stability to me begins with the one, thyself, nothing is stable only you can create your own stability. A relationship can add to the stability or take it away, they are so many differences and avenues involved, to each their own.........