Maxis has anounced the next base game update to The Sims 4 will allow you to pick your sexual orientation.
The feature will be available to all Simmers when the High School Years expansion drops. You won’t need to own the latter to add your orientation.
With Sexual Orientation, you’ll be able to assign a series of attraction parameters for your Sim. After you have it set, when a Sim whose gender doesn’t match your attraction settings attempts a romantic interaction, they will be rejected.