Is all marital foreplay moral?
No. Some acts of foreplay are moral, other acts of foreplay are immoral. Unnatural sexual acts, even between a husband and wife, are never justified, even if these acts are used as a type of foreplay.
Foreplay is a means to an end; acts of foreplay prepare for the act of natural marital relations. Since the end does not justify the means, every act used as foreplay must be good under all three fonts of morality, as those fonts spring up from each particular act. One act cannot be justified by the fonts of a different act. If an act is intrinsically evil and always immoral when done apart from natural marital relations, then the same act remains intrinsically evil when done with natural marital relations, even when used as a means to prepare for the end of natural marital relations. The end of natural marital relations open to life does not cause the means used to prepare for that act to change from evil to good. Nor is it correct to say that acts of foreplay have no morality of their own, but somehow derive their morality from the end. The idea that the means takes its morality from the end is merely a different way of saying that the end justifies the means.
Any sexual act that is intrinsically evil apart from natural marital relations, remains intrinsically evil when done before, during, or after marital relations. The end of natural marital relations open to life does not justify the means to that end. Nor can the good fonts of one act change the fonts of another act. An act with an evil moral object cannot be made moral by being combined in some way with another act.
If a husband and wife commit an unnatural sexual act (e.g. an act of oral, anal, or manipulative sex, with or without climax) as a means to the end of natural marital relations open to life, the means is intrinsically evil and always gravely immoral, even though the end is good and moral. The end does not cause the means to become good. The end does not change the morality of the means. The end does not exempt the means from the moral law.
Every unnatural sexual act is intrinsically evil, even within marriage, because this type of sexual act does not have both the unitive and procreative meanings joined in the same act. In order to be moral, each and every sexual act must be marital, unitive, and procreative.
Whoever says otherwise commits a grave sin by leading others into grave sin and by harming the holy Sacrament of Marriage.
More on marital chastity in my book: The Catholic Marriage Bed