Forced conflict is forced conflict.
Honestly, this means of insemination is very, very inefficient. Sex, besides not granting enough genetic diversity for the human species from 5 males, takes a long time, and men have long refractory periods. But you know what would be an effective solution?
You can turn somatic cells into pluripotent stem cells, essentially giving you an endless supply of gamete-producing variations. We have been able to produce
three-parent babies through similar, artificial means, and if you can produce endlessly reproducing gametes through somatic cells turning into stem cells, you can solve the reproduction problem relatively easily. Hell, I'm sure MC isn't prudish enough not to masturbate, so use the sperm he gets through ejaculation. (Albeit, sperm can't survive for long outside of the body, so this is severely limited itself)
The best part of the stem cell plan?
You don't risk cutting off genetic diversity...Essentially, we know there are several million men still on "ice" so to speak, and in vitro fertilization is possible. You can take the somatic cells, which can be taken from any shed skin, hair, etc. that the individuals MUST have to shed or dispose of to survive even when put on "ice" so to say, and use the cells (or even just the DNA found in the cell nucleus) to create pluripotent stem cells. We know that in nature,
very few males tended to mate with more females because men can produce more sperm in limited periods of time then women can produce eggs, which are a limited resource which take time to grow and develop, so the genetic diversity of humanity isn't in too much of a risk for future generations, though population will decline a bit.
But no, the mangaka just wanted to essentially create soft-core porn for us, which is a shame because I was really invested in the story until this last arc came about and detoured us away from the plot with some bullshit.