Because Kirk's tummy bulge is sexy!

Every so often on the Enterprise-A, there is something called a 'Cultural Immersion Evening', where various foods, exhibitions, and activities from different cultures are laid on for the crew to experience.  This time it is the Vindaloovian culture being examined. Captain Kirk and Captain Spock have retired to their joint quarters, to continue their immersion in private.

This is Fat Kirk/Spock by the way.  It's a bit kinky, but not awfully so.