Pleasure is a movement of the soul caused by the the apprehension and satisfaction of the sensitive appetites.
The passionate kiss that began as a show of love and led to arousal--knowing that the kids would probably interrupt and therefore not allow us to "conclude"--was a mortal sin?
Not at all. Pleasure is not bad. What is bad is to make pleasure from a means into an end. Pleasure should induce us to properly tend to the needs of the sensitive appetites. We are inclined to eat and drink because food tastes good and drink slakes the thirst, and we are inclined to procreate because sex is enjoyable.
It is perfectly fine for a married couple to kiss as a show of love and enjoy the pleasure of the kiss and the arousal it produces even if it is forseen that the arousal probably cannot be satisfied. It is absolutely wrong to kiss merely for the sake of pleasure and arousal with no other end intended.
A normal married couple that avoids sexual perversions like artificial contraception, masturbation, adultery, rectal sodomy and male oral sodomy needn't worry about this at all in their relations with each other.
The difference here, while I intellectually understand, seems to be very fine. You can enjoy the pleasure as long as you don't seek it? But we kiss because it is pleasurable. I cannot imagine having the time to consider whether I am kissing my husband because I love him or because I am ho . . .OOPS, excited, shall we say! The intermingling of these feelings in a relationship makes it hard to separate pleasure, love and, well, horniness. They are all there in varying degrees at any one time.