teachccd,
I’ll rephrase your questions as:
- in terms of morality, what constitutes an act?
An act is a knowing choice. Each and every knowing choice in human life is either moral (at least morally permissible) or immoral. It is never the case in any area of human endeavor that a set of choices is morally evaluated as one act. Each act stands on its own as to its morality. Now a set of acts might all be sinful, or might all be moral. But it is never the case, in any area of morality, that an act that is sinful to do on its own becomes moral as part of a larger set of acts.
Any unnatural sexual act is intrinsically evil because it is a sexual act (a deliberate use of the genitive sexual faculty) that is non-procreative and also not truly unitive. Most Catholics agree that for a husband and wife to commit such an act by itself is gravely immoral. The reason is that the moral object is evil. But committing essentially the same type of act (the same species of act), before or after natural marital relations, does not justify the act.
And the claim that all the knowing choices of a session in the marital bedroom merge to form one moral unity has been explicitly rejected by the Magisterium.
“Neither is it valid to argue, as a justification for sexual intercourse which is deliberately contraceptive, that a lesser evil is to be preferred to a greater one, or that such intercourse would merge with procreative acts of past and future to form a single entity, and so be qualified by exactly the same moral goodness as these.” HV, n. 14.
“But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good.” Casti Connubii, n. 54.
Every knowingly chosen act has three fonts of morality. One act may not borrow the fonts of another act. A sexual act which lacks the procreative meaning (an unnatural sexual act) is intrinsically evil, and it cannot become moral by borrowing the procreative meaning from a different sexual act (natural intercourse). Each knowingly chosen act stands on its own as to its morality.
- which types of foreplay are moral between a husband and wife?
The lengthly answer is here:
catechism.cc/articles/QA.htm#04
Notice that my answer is based on the three fonts of morality, and on the teaching of the Magisterium about the unitive and procreative meanings of sexual acts.
In summary, each and every sexual act must be marital, unitive, and procreative. Natural marital relations open to life is the only moral sexual act. Unnatural sexual acts are therefore not moral to use as foreplay.
However, other acts of foreplay, which are not sexual acts, are moral in marriage (always with three good fonts), such as kissing, embracing, caressing, etc.