ILLUSTRATION SHOWS SIGNS/DOMICILES OF THE LUMINARIES ARE DIVIDING LINESNot at all. I should probably post a pic of the Thema Mundi so that this makes more sense.
Here's the Wikipedia link for the Thema Mundi. It's supposed to be the birth chart of the world or of God. It probably isn't, but it's super, super, super old and what the domicile rulerships seem to be based on....or at least that's the way they were explained when the Greeks were laying this stuff out. It may be even older, but a lot of Greek authors fully articulated it.
It starts with the luminaries. The Sun gets Leo because Summer and all of that (if you follow the Tropical model) and the Moon gets Cancer because it's the domicile next to the Sun and the Moon is dependent on the Sun for her light. Then the planets go out in Chaldean order from there: Mercury, Venus, Mars, Jupiter, and finally Saturn.
The signs of the luminaries are dividing lines.
Everything from Cancer to Capricorn is the lunar half,
everything from Leo to Aquarius is the Solar half.
From this you can probably see why Jupiter's trine to Cancer
matters in the domicile assignment but Mars' trine to Cancer form Scorpio doesn't.
The domiciles of the luminaries are dividing lines.
In other words, as far as the rationale for the domicile assignments are concerned, it's incorrect to say that Mars trines Cancer from Scorpio as it has to cross over Leo to do it. What's more immediate is the fact that Scorpio squares Leo, the home of the Sun. Squares are discordant and Mars is the lesser malefic. Aries squares Cancer, so that's Mars' sign too. The fact that Aries trines Leo is irrelevant because it has to square Cancer first. Mars is seen as bad because the Sun is the "life force" and for us humans our soul and perceptive capacity. The Moon is the physical body and is tied to physical well-being. A planet in conflict with those basic things isn't necessarily easy to get along with. You get the idea.
As long as you see that aspects can't cross the domiciles of the luminaries, then everything falls into place. I'm also not defending the absolute perfection of the Thema Mundi and especially not Ptolemy's seasonal rationale for the domicile assignments. I'm just saying that this is what it's all built off of.
As for Scorpio being fertile....yes and no. I should've been more clear. Ptolemy, when providing his seasonal rationale for the domicile rulerships, was talking about the time of year being fertile or infertile. If we follow the seasonal logic Cancer is a very fertile time of year in the Northern Hemisphere. It is the very start of Summer. Pisces less so, but it is also when the snows melt...thereby setting the stage for the growth of Spring. Scorpio however.....well no. Scorpio is the middle of Autumn, where everything is dying. I believe Bonatti (writing waaaay letter) refers to Scorpio as poisonous water. There are issues with this seasonal rationale of course, but that's the line of thought Ptolemy was going on.
Scorpio is fertile in another sense. This is the sense of fertility I believe you were talking about, but this doesn't have anything to do with seasons. The so-called fertile signs were originally called "Aquatic" signs as they all represent animals seen as aquatic (Scorpions aren't actually aquatic, but meh). They are "fertile" in the sense that they indicate having many children as the animals that they're based on all have many, many children (Crabs, Scorpions, and Fish). This has nothing to do with season and everything to do with the image of the constellations. Scorpio indicates having many children but is pretty awful if you follow the seasonal rationale.