My review is based on having read 7 chapters (that was all I could stand). There are some slight spoilers.
It seems like this manga was created solely with intention of creating the most spineless MC possible. I'm used to wimpy MCs, but Mizuhara Reito is on a whole 'nother level. It's like the author has some kind of extreme paraphilia involving men being absolutely useless. Mizuhara's spinelessness ventures into the realm of delusional.
The story begins with our "hero" confessing his love to Elisa. The impetus for this being he is diagnosed with a terminal illness of some sort. But all is not lost. All he has to do is go into cryosleep for 2–5 years while an AI figures develops a drug specifically tailored to him. Needless to say when awakes things are not as they should be.
While he slept somehow a virus has killed all the men in world. The only ones left alive are those who were also placed into cryosleep. However, of those in cryosleep only those who were specifically being treated for whatever illness befell the MC are immune to the MK (literally Man Killing) virus. The other remaining men who have survived in cryosleep will should they awake. It is therefore Spineless MC-kun's job (along with the 4 other random people who just happened to have his exact circumstances) to impregnate as many women as possible to ensure the future of the human race.
So the situation seems pretty dire, right? The reasonable thing to do would be to politely start impregnating women, right? MC's handler even went though the trouble of finding fertile attractive who want to have sex with them. It wasn't like she just went around with a picture of him and asked "u wnt sum fuk?" to every girl. No, she constructed rigorous surveys which featured pictures of other random men in addition to the MCs. The MC received favourable results. From this selection of women who would consent to be impregnated by the MC only the most genetically fit were selected. Resources like food are also scarce. Bearing and raising MC's children is basically the only way to ensure a stable lifestyle. It makes sense to prioritize the allocation of resources towards those who are helping to guarantee the future of humanity. We've got hot, nubile women, who aren't being overtly coerced or anything, waiting for MC to come give them some dick so that they can secure a future not only for themselves, but also humanity, and what does the MC do? He rejects this offer, saying he is saving his dick for Elisa (who has conveniently gone missing during the time he was asleep). Rather than having sex with all these girls, he is going to single-handedly cure the virus.
There are so many problems with that setup. Firstly, despite being around Elisa for 10 years straight, Spineless MC-kun only manages to confess when the possibility of him potentially dying comes up. It's even an incredibly weak confession—in the translation I read the MC simply says, "I like you". In the future world MC inhabits working isn't even necessary. Despite this he pledges to become a doctor along with Elisa in order to fulfill some stupid dream. Despite this "I like you" is the best he can do. Because of his absolutely tenuous commitment to Elisa, he won't have sex with all these other women—despite the fact that it absolutely benefits everyone. He additionally claims that his dick doesn't matter because he could only impregnate about 100 women per year. If he only had sex once per day, everyday, with women who were ovulating, 100 women is about the number you'd expect to get pregnant (assuming the chance of pregnancy 27–33% during ovulation). But why would he be doing so little dicking? His research can't take up that much time; or is it only possible for him to have sex with 1 woman per day because he needs 8 hours of foreplay in which his partner has to reassure him that he is a nice, special guy, before he can get hard? Also, how does he expect to develop a vaccine for the MK virus, when all the other lady-scientists who have been working on it for the last 5 years have failed? He's only a medical student. Does his dick somehow make him more capable? If we look at the manga, that may actually be the case. Apparently all the technology has broken down because all the male engineers died (thus leaving only female engineers). In the year of 2040 wear working is optional, they apparently haven't figured out how to get more women into STEM. The one's do get into it are apparently beyond incompetent. I think the manga , actually mentions them not understanding the principles behind the technology. So not only are they stupid, they are apparently incapable of learning. I never expected this manga to be a shining paragon of feminist virtue, but this just seems like such an absurd way to be sexist.
Shuumatsu no Harem is just terrible. It is actually probably the least erotic I have read, that isn't trying to deliberately disgust or horrify the reader through its use of sex and nudity. It's actually just incompetent. I didn't think it would possible to take such a lurid and easily exploitable setup, and then make it incredibly boring. Sure, there is a fair bit of T&A, but watching Mizuhara's superlative spinelessness is sufficient remove any enjoyment gained through seeing naked anime girls.