The intention (or purpose) to use one act as a preparation for a moral act of natural marital relations is not sufficient to justify that preparatory act (i.e. that act of foreplay).
Says who, and exactly what, and where? By your interpretation, no foreplay of any kind whatsoever would be allowed.
The Magisterium teaches that, to be moral, each act must have three good fonts of morality:
- the intention or purpose for which the act was chosen.
- the moral object (intrinsically evil acts have an evil moral object)
- the circumstances, especially the good and bad consequences.
So a good purpose, by itself, cannot justify any act. Foreplay is only moral if it has three good fonts, not only one.
As for non-procreative sexual acts, these are explicitly forbidden by Catholic teaching:
USCCB Catechism: “Each and every sexual act in a marriage needs to be open to the possibility of conceiving a child.” (p. 409)
This is talking about acts leading to climax for the man; not foreplay. Nowhere is anything forbidden if it is foreplay (except for where the man climaxes), leading to intercourse.
In Catholic moral teaching, an act is a knowing choice. Each decision, each free choice, each deliberate choice is a separate act. Each act must be evaluated on its own merits as to its morality.
Catechism of the Catholic Church: “When he acts deliberately, man is, so to speak, the father of his acts. Human acts, that is, acts that are freely chosen in consequence of a judgment of conscience, can be morally evaluated. They are either good or evil.” (CCC, n. 1749).