Should've kept that marisa alive and give Marisa and Reimu one of those plot device medicine that makes yukkuri borderline invincible and lock them up in a cage and watch the shitstorm
The reason I came here, Reimu, is you. WE came here for revenge.... yu?Well Marisa, best not get involved in a couple's quarrel.
This seems to be a play on the common expression "夫婦喧嘩は犬も食わない" ("even a dog wouldn't want to eat a lovers' quarrel" = "one should not interfere in lovers' quarrels"), replacing "dog" with "Momiji".