Note this is postulating that this starts from Stone Age.
'Mate' is defined as penile-vaginal sex with ejaculation.
Requirement starts at puberty. People know when it starts, how long they've got, the general info that this death-clock resets by sex, and they are also killed, via magic. Cannot be circumvented by science. Inter-sexuality does not exist, as I can't figure out how to rework the mating condition to prevent that from being an automatic death sentence.
Each mating gets each partner 30 days of life. Unless it's a woman, whose pregnancy keeps her alive for the whole pregnancy, and 30 days after birth.
Mating with condoms or any other kind of physical contraceptive, grants 1/2 the time to live of the previous mating. Like a 30-day girl and 30-day guy mate, and then they both have 15 days to live, keep doing it, and they could get to a point where they can't mate fast enough to prevent death.
However, non-physical, non-immediate contraceptives, like the Pill, or the rhythm method, or douching, don't shrink the 30-day death clock.
But, Inter-Uterine Implants or any kind of Surgery, do shrink the death clock.
Note that if a male gets a girl pregnant from mating, and they still get 30 days from that mating, but if they mate with her again, on their 16th day, then they just get one extra day of life. So, it's best to space it out farther apart... And there's some math like... 30+15+7.5+~4 +~2 + ~1 = ~59 ... If you time it so they cut it really close, they get a little bit less than ~2 months before they have to switch to a non-pregnant girl.
That's also how it works with normal sex. If you have non-pregnant sex on consecutive days, you keep adding only 1 day to your death clocks. as 30 days from day of last sex is 1 day before the last time. The death clock counts from day of last mating, and resets at each mating.
Also, women don't run out of eggs.
Effects of males mating with pregnant females is the same as contraception-mating.
People can die by injury and illness, but not of old age. Immortality begins at 20, but the 'immortals' are still subject to Mate Or Die.
Infertile people function basically as normal in the universe, as long as males can still ejaculate, and women still have a vagina, so certain injuries will kill by proxy.
Lastly, for now, if you're on life support, you still need sex, or you'll die.