Mary,
It only lends itself to those thoughts for you because your understanding of the expression of the sacrament is in metaphysical terms, that being Transubstantiation. Now, this is not a criticism of you or any other Catholic. Its to be expected.
Sacramental union, OTOH, is not in any way a metaphysical construct. It isn’t meant to be. The use of the terms “in, with, and under” are not intended to imply at all a consubstantiation (co-mixing, or new substance out of a combining of bread and body).
Herman Sasse: *** It is impossible to define Luther’s doctrine as consubstantiation. Even the words ‘in the bread’, ‘with the bread’, ‘under the bread’, or ‘in, with, and under the bread’, were never regarded by Luther as more than attempts to express in these old, popular terms inherited from the Middle Ages the great mystery that the bread is the body, the wine is the blood, as the Words of Institution say. ***
As you see, the expression is “in, with, and under”, the doctrine is “this is my body”, etc. And there is a parallel here to the Catholic use of Transubstantiation, that being to find a way to express the mystery in such a way that the faithful may have a form of understanding to grasp regarding Christ’s words, “This is my body”, etc.
This parallel seems quite enough evident that Lutheran and Catholic theologians in dialogue have said this:
prounione.urbe.it/dia-int/l-rc/doc/e_l-rc_eucharist.html
Jon