I think they held this pose for their cuckold’s benefit: to demonstrate how far her boyfriend’s cock reaches inside her.

To make it more interesting, they could do something I’ve done with couples: have her press her boyfriend’s cock firmly to her belly and have her cuckold use a Sharpie to draw an outline of her boyfriend’s cock on her. For even better illustration, use a different color Sharpie and repeat the process with her cuckold!

Result: a penis nested within a cock!