In terms of such signs of affection such “unitive acts” that are sexually stimulating.
One will find as I recall various judgments regarding this matter. Even among orthodox Catholic moral theologians. The Church does not get per se into detail in this area.
Certainly if such leads climax not continuous with the marital act (husband only during intercourse) or to other serious sins mentioned it is gravely disordered. Such comes from the Church’s Teaching in terms of the Marital act and in terms of masturbation etc.
If it is a near occasion of the above (without some necessity etc) such too “can” potentially fall into the grave area. Such is applying the understanding in moral theology in regards to the near occasion to mortal sin. Hence too one can find mention of avoid that which is “vehemently” arousing. Again this is mortal theologians applying principles.
One can also note the “frustration” aspect. So that too can play into considering a particular act.
I think too that certain very directly stimulating actions are in the least are to be avoided when such is not going to be “foreplay”.
So can certain unitive actions such as passionate kissing be possible in marriage outside of foreplay? Yes…but one needs to guided by prudence, chastity and charity.
As to during times when one has a serious reason to abstain for a period of time…well one needs to be particularly prudent (and it may be that one need not abstain due to near occasion of sin anyhow…depends on what the serious reason is in that case…such is part of the NFP use to avoid judgment). Here is a recent article that gives the take of two theologians
zenit.org/article-33689?l=english – note that they say “it seems to us”…I imagine that various discussions among theologians as to various aspects of the question can take place…but in general certainly I would say if it is a “near occasion” and one “must avoid relations” then I would avoid that which starts the engines too much for sure…
( PS: As to NFP. The term the Church uses more often in documents in recent years is “serious reasons”.)