Power differentials are an issue insofar as there's cohersion and manipulation involved, especially if the other person can't just say no and has to accept.
In real life, this can be a murky and complicated subject, with considerations about what could be lost from rejection, but in my completely honest opinion, ultimately these are adults we're talking about. Their decision to engage in intercourse with a power dynamic, so long as it's not literal r*pe or SA, is theirs to make.
The fact this is a video game with explicitly programmable behavior means we can define the lines more clearly than in real life. I think the fact that a follower can say no to whoever they choose (and the supporters here are in consensus of that being a part of this as well) does mean that followers can consent to mating, whether it's with another follower or the leader. A follower is, by statistical probability and game design, not obligated to mate with you and there is no mechanism to coherce or seduce them into it besides the lust ritual, which applies to literally everyone.
which could also theoretically include cultists that were raised since birth by the leader and thus would add grooming into the game as well.
If that's such an issue, easy fix really. Don't let Lamb mate with followers who grew up in the cult.
On a broader note, I don't really see the point in arguing about moral consistency. This game, as I see it, is very inconsistent when it comes to moral bars. That's why you always have (pardon my french) shitheads who go "lolol u can sacrifice and eat followers sooo why can't we have incest?"
From what I've seen from dev decisions it's a matter of what's considered acceptable to portray. And things like sacrifice, cannibalism, brainwashing, and yes even egg cracking are kosher because they aren't major societal issues. I guarantee you if sacrifice and cannibalism were hot topics in real life, or if eggs were literal fetuses, we wouldn't see them in game.
The question then is, is Lamb mating with followers something that would become a hot topic. I don't think the vast majority of people would look too deep into it honestly, but that's just me.