And I'm not saying its justified. I'm saying that they had a choice between lining up all of their POWs and executing every Nth one on the spot, allow their soldiers to use the narcotic qualities of their venom to instill addiction, or to torture their POWs for information and test to see which aphrodisiac or even vesicant gas is most effective. High Command is poised to, and likely going to, denounce the act of taking trench wives once the war is over.It's both. They wanted to win the war so they decided they would be super evil. You cannot excuse any and all actions just because it's a war. Our own history is full of horrible atrocities that were committed during war time that no one, even the people fighting in the war then, see as justified. Drug addicted sex slavery cannot be justified with "but we REALLY want to win, guys!"
Additionally, it's not like the venom even makes the victim incapable of thinking for themselves. Sierva, one of Lieve's trench wives, can approach you as you're on the path between Kressia and No Myr's Land late in the day and ask you to take a message back to her lover that she had in Gildenmere. (That is, the one she had before she got left for dead on the battlefield and subsequently rescued by Lieve.) If you ask her about what Lieve thinks of this, she'll beg you not to tell her, because she's not supposed to send letters back home. (They could possibly be military secrets, you know.)
Moreover, both sides use red venom for their own purposes. You can run into a group of lustful Gold myr soldiers while wandering around Gildenmere, if you agree to follow them, they will fuck you for hours on end, giving you multiple doses of red venom because your refractory rate doesn't allow you to go every two seconds.