Dearest Life
This is based on a prompt found in the AO3 Facebook.
Sherlock is an Omega whose Alpha has died severing their bond. Sherlock is depressed for an Omega who lost their mate is considered worthless in their society. If young and fertile, unbonded omegas are sent to breeding camps, forced to bear pups they will never raise; something his brother, Mycroft, will never let happen.
Or there is the option of becoming the Second Mate of an Alpha who is already bonded. The omega would be welcomed into the home, bear all the pups the Alpha wished and live comfortably and protected. There is no love and the second's mate place is very low compared to the spouse, but it was better than the alternative.
Still mourning his beloved Alpha, Sherlock refuses to be someone else's spouse, let alone a lowly Second Mate, but time is running out before he’s forced to go to the camps. Sherlock is forced to marry John Watson, a wealthy Alpha doctor, who is married to Mary, an infertile Beta. Both of whom hide dark secrets from each other.