The article misses the entire point of civil marriage in the first place, which is to provide incentives for a structure historically seen as positive for society, not to give individual people what they want. We provide (dis)incentives for all kinds of activities, of which marriage is one. Charitable giving and smoking are a couple other areas we try to nudge behavior with tax policy.

The authors are people in one sphere of society saying they don't like laws that incentivize behavior that is different from their own. But nevertheless, they are making a conscious choice (even with information about the additional economic costs etc.) to remain unmarried, which includes associated costs and everything else that comes with that choice.

In their case, we all win in one sense. Society gets some additional tax revenues to support social services, and the authors get something they prefer instead: singleness.

Not all of our preferred behaviors are given incentives by the government; only the ones considered compelling and in the public interest. Civil marriage primarily exists for the state; not really for you.

Note: Sure, there are good and interesting arguments about the social value of civil marriage, whether or not it should even exist, and if so, in what form. I'm not going there. I'm just pointing out that the authors are making a conscious economic decision and are disappointed that they pay an added price.