It pops out

The movement of their fall, combined with the sudden brush of Sophie's hand, has caused Rod's shorts to slip slightly. With your added power of arousal, it's enough to cause the head of Rod's cock to slip out the top of his shorts, Sophie's fingers accidentally brushing against the exposed tip.

Their eyes lock as you plant more ideas in their heads, and Sophie's fingers caress the tip for a few seconds, before snaking around his cock, gripping it firmly and pulling it further out of his shorts.