Could be a few things:

• one is clever cuts. Film one scene and film a scene at a different time. The time you see the actor looking at something erotic, you position the camera on them but they are looking at something that is not erotic so they don’t get aroused. But you give the illusion that the character is looking at it to the audience.
• masturbation. If you masturbate before the scene, then you are less likely to get visibly aroused.
• get an actor who isn’t aroused by that. If filming a scene with a man trying to seduce another man and he fails, then get a straight guy who won’t be aroused by the gay seduction.
• too many times filming. Many scenes you see on TV and movies have been filmed multiple times. Rare is it for them to do one cut and be done, even if it was perfect and got everything the director was hoping to see. After doing that scene a few times, the natural feelings get lost on you. It’s why a lot of funny scenes that make the audience burst out laughing uncontrollably can be filmed without the cast and crew doing that to ruin the scene. You’ve done it so much that you are now so tired and bothered, you don’t feel the natural urge to laugh or be aroused. Some directors will use this to their advantage in, what some may consider to be unethical, to agitate their actors into feeling emotions for a scene or so they don’t feel emotions for a scene.
• some actors just have the natural ability to hold back some natural emotions. It’s what makes them so good at their job.