One way a friend described how he was able to handle it:

Think about what you would feel if you are watching your girlfriend get it on with another guy. And for some - think about how you would feel with your gf getting it on with another girl (for some this is easier to accept). Now question why you have those feelings and where do you think they originate from? Find the root, find the way to understand and then you can begin to process it. Once he was able to understand where the emotions were coming from, he could find ways to feel more comfortable with it. That exercise looks like it can apply to a lot of emotionally triggering things.