No sin at all, in my opinion.
As for whether the leak is merely lubricant or actually contains semen, this is morally moot. There is no intention to sin - that is, the couple is not arousing themselves in order to deliberately ejaculate in a non-procreative manner. They are merely expressing affection with arousal as a natural unintended effect (based on the OPs explanation).
While it isn't sinful, IMO, I'd still suggest it's reached a point where it's unwise. Once the male is so excited that he's leaking, there's a couple of potential problems: a) it can become hard to stop once arousal reaches a certain point, so this sort of making out could become a near occasion of sin for any given couple and b) when a male gets that excited and doesn't ejaculate, it can become very uncomfortable - sorry for getting graphic, but "blue balls" is a geniune problem that should be avoided. Many couples who need to abstain for a time (postpartum, during NFP, etc) come to the conclusion that it's better to avoid too much intimacy of this manner because of this effect (which can apparently affect females too?).