To the OP- Always talk to a competent director/confessor
Yes, it is gravely sinful to engage in “foreplay” which is a preparation for sex, without intending to finish the act with the man climaxing inside the woman. It is sinful to do this because it is separating the unitive part of the marital act from the procreative part, in giving you pleasure without opening the door to God giving a child.
If you and your spouse wish to get intimate, you must do so with full intention of finishing the act. If you intend to go “just so far” because you are at a peak part of your period, and then you intentionally do not finish the act properly, it is a sin.
The above isn’t entirely true. Any touching is allowed within marriage because even if it is not in immediate preparation to the marital act, it is part of a lifetime of marriage which includes many times in which intercourse happens. So its maintaining intimacy during the fertile time in anticipation of the infertile period.
Here is what the moral theology book by Fr. Herbert Jone has to say on the matter. His book is a very traditional and faithful work and therefore is certainly a good and trustworthy source:
“*Imperfect acts that are sexually stimulating are lawful or forbidden according to the following rules:
…
2. Apart from the marriage act which the couple either cannont or do not desire to perform:
a) mutual acts that are
sexually stimulating are lawful when done with a
just cause (e.g. as a sign of affection) if there is no danger of pollution (
even though this should sometimes accidentally follow) or even if there is such a danger but there is also a reason justifying the action (e.g. to avert suspicion, to restrain one’s consort from adultery, in obedience to a request).
What is said of the danger of pollution holds if the acts naturally tend to have such an effect; not however, if such an effect is only accidental e.g. on account of the peculiar disposition of a person. Actions apt to result in pollution are grave sins if performed without a just cause; actions unlikely to have this effect would not be more than venially sinful, unless performed with the intention of pollution. *”
Pollution means finishing unnaturally.
Bold emphasis above is mine