Indeed that’s a valid point. Divorce rate has hit the roof. But I can assure you that traditional marriage has nothing to do with why people divorce.

And if we implement the ideology you posit, can we be sure that divorce rates would go down?

And even if divorce rates do indeed go down, can we be sure that people are staying together for the right reasons?

About your last point, I agree that both kinds of marriage have their disadvantages and advantages.
There’s no ideology or ism that’s without flaw.

However, most times the problem isn’t the ideology but us humans practicing these ideologies.

Take capitalism, individualism and liberalism for example, these are founded on great premise.
But today, the result is disastrous.

Because we’re good at abusing opportunities.
Just as we’ve abused monogamous marriage.
And am certain, we’ll also abuse polygamous marriage if we follow your opinion.