There are many reasons and they don't all always apply to every cuckold. For some, it really is a simple as wanting to witness their partners pleasure from a distance (even if that distance is only 2 feet).
For some, it is much more complex. For some, it is the desire to be dominated. How better to demonstrate complete dominance over another than to flaunt your "infidelity". Well, except to have your partner clean you after you're done.
For some, it is a desire to be dominated thus. For some, they would rather it be a sign of their submission.
There are also those who harbor hidden (sometimes even from themselves) homosexual fantasies, and this is a way to experience that side of themselves without taking responsibility for their actions.("Hey, She MADE me do it, so I must not be gay, right?").
I also imagine there are those who really don't do it for their spouses and, rather than risk losing them, "allow" them the freedom to seek satisfaction elsewhere. Some of these try to share in the experience, some don't.
Another element to this is that we also tend to eroticise our fears. It helps us deal with them and face them.
Please don't take this list as definitive. There are as many reasons, I'm sure, as there are cuckolds.
FUCK MY WIFE!
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