The world of Alphas, Betas and Omegas. Everyone is born either male or female, but they have a second sex. Betas are normal, Alpha are seen as the elite and have both heightened physic and intelligens, while Omegas are often times seen as the trouble of society. While all females of all the three second genders can conceive, so can Omega males. Omegas are most fertile during their heat and can often send an Alpha into rut. This is noticed through the secretion of pheromones.
To make this even more "romantic" there are fated pairs. Although this only applies to Omegas and Alphas, it's not always sunshine and rainbows. Then there are bonded pairs where an Alpha has bitten an Omegas nape during their heat. This is also not always romantic as this can happen even when the Omega or Alpha are unwilling.
Enough of the explanation, please enjoy the different dynamics in the omegaverse.
Also TW there are often instances of nonconsensual intercourse, violence, and discrimination against second gender.