If we are discussing the intrinsic meaning of a Civil Union then in principle (like adoption) the exchange of bodies is not intrinsic to the meaning of the institution.
What you refer to, and wisely so in many situations, is therefore a separate but “concomitant” prudential consideration - the desire to simulate marriage which, for Catholics at least, intrinsically involves the exchange of bodies.
We must therefore be open to prudential possibility that a Catholic couple are chaste in a Union situation and are seeking legal/familial rights similar in nature to adoption.
If that is the case then there is nothing in the nature of the institution itself that is disordered.
That would not be the case for a State SS “Marriage.”