This has lower inbreeding risks than serial monogamy and polygamy but higher risks than lifetime monogamy. This is what I was thinking about when I was initially thinking about the aliens and how they would prevent unwanted pregnancies without temporary sterilization. And unless the male in the pair dies before the female is 50 years old, it would most likely be the male that is able to go from 1 monogamous relationship to the next and not the female. It also has a much lower chance of people with genetic disease sneaking out of the sterilization system. This, I view as the ideal because this has the lowest inbreeding chance and lowest genetic disease rate. In any kind of monogamy, the birth rate would have occasional dips and rises but overall be stable. Lifetime monogamy is where it only changes if 1 of the partners dies. Irregular monogamy is where it changes but the number of cycles before the change varies. Serial monogamy is where it changes with every cycle. Monogamy can be divided into 3 types based on whether or not it changes from 1 cycle to another. On the other hand, birth rate would be higher than that of polyamory and might even be higher than that of monogamy. Females are more likely to cooperate than to fight. The only issue that is gotten rid of is fighting. Polygyny is slightly better, emphasis on slight. But there might be some babies with genetic disease that sneak through the system if all relationships are polyamorus. And females could induce lactation if they really wanted a baby. Females will still have periods and males will still have increased muscle growth. Now of course, if a baby is born with a genetic disease, it will be given a complete spermicide at a safe age if it is male and ovulation inhibitors(LH inhibitors to be more specific because LH is the hormone that causes ovulation) if it is female. Plus, genetic diseases will spread much quicker this way. This will also lead to more inbreeding because now there is more genetic similarity among all humans. The males could easily get into a fight and make the single mom have to defend herself and her children.
All the children from a given group will have the same mom and some will even have the same dad. This is in my opinion, the worst type of sexual relationship. Both could lead to an eventual die-off of the humans but in different ways. Polygyny is where 1 male has a sexual relationship with multiple females. Polyamory is where 1 female has a sexual relationship with multiple males. Polygamy can be divided into 2 types based on predominant sex. I am talking about polygamy vs monogamy and what would likely happen.
And I am not talking gay vs lesbian vs straight. I was thinking about what could happen with different kinds of sexual relationships.