One common trick is to have the guy shoot the particles, then some cum clothing she was already wearing suddenly becomes visible. You set the scene up with the cum clothing all over her, and set the Alpha to -1. When he comes, whatever triggered that can cause the Alpha on all the cum clothing to go to 0. Of course, then you need a Reset or Clean button to clean up the mess setting the Alpha back to -1. The trigger can make the cum clothing appear slowly, or it could appear in stages, probably using Logic Bricks.