Assassins are People Too (Assassins in Love #1)
S.C. Wynne

Instantly, Marc and Dillon develop a strong sexual connection. As the months go by, while they steal time together, it develops into something more. But Marc doesn’t really do relationships, and Dillon worries Marc will never be able to love him back the way he needs.
Also, when you’re a paid assassin, not everybody is rooting for your happy ever after.