Nothing makes me more aware of my status as a sissy cuckold as putting his cock inside her. You might think that sucking her lover's cock or eating his cum from her just fucked pussy would be more meaningful but not to me.
The act of putting another man inside your wife is at once exciting and so humiliating. It strips away your status as whatever kind of man you once thought you were. Your wife or girlfriend, if she's understanding will assure you of her love even as her lover fills her in a way you never could.
There is nothing more emasculating than this one little act and yet I love to do this for her whenever I am given the privilege.