Normally, Alpha!John dates Beta women, but one day he starts dating a younger, good-looking, interesting male Omega. Omega!Sherlock is panicked and jealous. He tries to find faults in this Omega, but the younger man, while he's not perfect, has nothing really bad about him and is truly interested in John as an Alpha and as a person. Maybe the Omega is nice enough not to complain to John when Sherlock insults him really badly, because he knows Sherlock is John's friend and is important to him. It doesn't mean he's going to break up with John though.

Sherlock tries to ruin their relationship, it was never very difficult with all those Beta women, but it doesn't work, and maybe John is really angry with him to boot. Sherlock deduces that John contemplates bonding with the Omega, but probably isn't 100% ready yet, so they just date, have sex and spend the Omega's heats together ('What if the Omega gets pregnant?!', 'What if John gives in and just bonds with him?!'). Distraught, Sherlock has to face his own feelings towards John and admit to himself that he does want John as a bondmate. Sherlock's very nature rebels against the rival, so he goes into heats despite his suppresants and not when he should have gone into heat if he wasn't taking them. 221B is saturated with the rivalling Omega pheromones.

Good John/Sherlock ending is preferable, as well as virgin!Sherlock, who had never wanted anyone before John.