I’m pretty sure the cum originated from their Balls rubbing and grinding together. I can imagine the accelerating sensation it will cause on their cocks and very soon there will be Cum.
Imagining my lips around those cocks as they shoot…hard as a rock now! Used to frot with a buddy…and cumming at the same time was THE BEST feeling. Afterwards he’d drop on top of me and we’d kiss and savor the feeling of our loads between our bodies.