Man Hunt by Daniel May

Diomedes ‘Dio’ Alms is the middle child of the famed, feared Alms criminal dynasty. Content to spend his time jumping horses and gallivanting with the ladies, Dio skates under the radar of mob politics… until he doesn’t.
Mars, an assassin with a penchant for lipstick who has already made Dio question his sexuality once, pops up out of the blue with a proposal. Well, more of a demand. He wants to be Dio’s plus-one to the most exclusive event of the year: the fifty-second birthday of the universally feared William Izawa.
Their fake dating goes off the rails as soon as they arrive. Izawa’s birthday bash centers around deadly games and the guest list is littered with ghosts from their paths. Worse, the cold-blooded Mars faces a very real danger of falling for his hot, hapless hostage, and Dio must tackle a philosophical question for the ages:
Is it really gay if the other guy is disguised as your female fiancée?