Our Gemara on Amud Aleph certainly qualifies as one of the more surprising Gemaras in Shas.  Given the intense rhetoric regarding wasting seed in the Gemara, such as Niddah (13a), Kesubos 46a, and Avodah Zara 20b one should see nothing positive in a seminal emission.  Indeed, even this Gemara is ambivalent, noting that one should first worry about surviving the year.  Only after surviving the year, is it possible to see positive portents.  Let us take a closer look.

A tanna taught a baraisa before Rav Naḥman: With regard to one who sees an emission of semen on Yom Kippur, his sins are forgiven...The school of Rabbi Yishmael taught: One who sees an emission of semen on Yom Kippur should worry the whole year that perhaps he was given a sign that he and his fast were rejected. But if he survives the year, he can be assured that his good deeds protected him and ensured for him a share in the World-to-Come. Rav Naḥman bar Yitzḥak said: Know that it is so, as the whole world is hungry due to refraining from conjugal relations, and he is satiated since he emitted semen and his lust was subdued. Since the issue was involuntary and not intentional, it is a sign that he has merited divine compassion. When Rav Dimi came from Eretz Yisrael to Babylonia he said: Seeing semen on Yom Kippur is a sign that one will live a long life, grow, and raise others. An allusion to that is the verse: “That he might see his seed and prolong his days” (Isaiah 53:10).

Let us try to understand this Gemara better, based on the commentaries:

Rashi explains:

This is a good sign that he will have children and live long.  Rashi later explains that if he was given such a “satisfaction” while others were in a state of deprivation, either his efforts at fasting are being thrown in his face and rejected, or it is a sign of unusual merit.  Thus, if he survives the year, he must be a completely righteous person. Furthermore, some interpret the idea as his deprivation is not required as his sins are already forgiven (see Magen Avraham OC 615, “muvtach”). The Machatzis HaShekel (OC 615:3) quotes the Peri Chadash as arguing on this point. This is not evidence that he is a completely righteous person, as how could a truly good sign come from impurity? Rather, by dint of the fact that he SURVIVED the year, he must overall have a great many merits that protected him. 

Also of note, Mishna Berura (Op. Cit. 2) quotes authorities that maintain the entire case of the Gemara is referring to a seemingly spontaneous emission with no sexually arousing thoughts earlier in the day. When it occurs in this fashion, it is easier to attribute this an ominous portent since it seems to have no natural cause. But if the person had sexually arousing thoughts during the day, then we attribute the nocturnal emission as coming from that, and it has no significance beyond the sinful behavior of the erotic thoughts. Likewise, Machatzis HaShekel (615:2) says the same idea applies to unusual physical exertion or eating certain foods known to increase the amount of seed. If we can attribute the nocturnal emission to natural physiological causes, the omens of the Gemara do not apply.

I will conclude with a lesson by looking at the meta-aspects of this Gemara.  In the end, we see that nothing is absolutely impure or improper.  Thus, even seminal emissions which are considered a severe impurity and often sinful can also be seen as a positive sign and blessing. Ultimately, it is the source of life.