There is also another reason to be highly against the Dark Ritual. Making Kieran shoulder the burden of housing the Old God soul without him having any say on it is no different from Dorian's parents wanting to force him to be someone he is not and his father was willing to use blood magic to make that happen. Recall that Morrigan and the Warden could potentially engage in a blood magic ritual to force their offspring to lead a life that he did not ask for. In many ways, it is no different from parents who treat their children as merely a tool to carry out their dashed hopes and wishes instead of a sentient being that should choose their own path someday.
That is probably the biggest reason of me being against the Dark Ritual. I was the firstborn of an Asian family, the son who was shouldered with all the wishes and hopes of his parents and as such, was never really allowed to choose a life path he wanted until I rebelled and we had massive arguments where I won. I do not want an innocent child to be shouldered with the burden of carrying the soul of a powerful ancient being without him having any choice in it, much in the same way I do not want anyone to be raised and driven to study solely to be a medical doctor that gets married, gets a big house and pops out the typical 2.5 kids and have hobbies that his parents had to approve and control, has to eat food that his parents had to approve and control, etc.
More importantly, I also do not see how one can choose to remain in love in Morrigan and not do the Dark Ritual. She effectively betrays you by leaving her love at the eve of his most important battle because the man she claims to love, the very same man that she could not manipulate, has some shred of ethics. It did not matter if you helped her by killing Flemeth and retrieving the real Grimoire for her. It did not matter if you were able to see past her acerbic nature and her other flaws and loved her. What mattered to her was the Urthemiel's Soul and when she is denied access to it, she throws up a tantrum and leaves.