you take the sitaution in the wrong order.Recall that SSM is now widespread. In that context - can the existence of civil union laws do harm? Civil unions would be an improvement (in our eyes) compared with SSM. Of course, there is likely little to no demand for them. The gay community see them as 2nd rate and rejected them in the past.
In many, certainely the majority of countries, where civil unions were created it was before SS marriage. Not in the other order.
And in some countries thoses civil unions are also for all “couples” regardless of the sexes of the partners.
civil unions are a step to prepare people’s minds. When same sex unions are normalized in the population it may be easier to reform the meaning of marriage. But not always as strong opposition happened sometimes (like in France).
Civil unions don’t necessary end with SS marriage. It is true when they are also for heterosexual people.
But what these unions definitely does is to weaken marriage. Couples make a civil union before or instead being married. It offered some advantages such as “exit” option on demand without having to divorce. Goods of the partners are not necessarily become into community. The partners are not bound to provide for the needs of the family “in law” members in need. There is no to little inheritance rights (which preverve the rights of the children or other family members).
We cannot said that theses unions and their consequences does any good on the stability of families and as a consequence society at all.