Not true.
While many cuckolds are "penile challenged", I know a few cuckolds that have very large dicks and many that are at least average like myself. The one thing we all have in common is a desire to submit to our Wives desires and make them as happy as we possibly can.
Many women enjoy having the freedom to have sex with a variety of partners, regardless of the size of their own husbands cock.
Some women enjoy interracial sex because of the taboo element or because the enjoy the more dominant sexuality that many black men exude.
Some women are married to men with large dicks and prefer to have sex with someone that has a smaller endowment so that they can fuck longer without becoming sore.
There are many reasons why a woman may enjoy having sex with other men and women, however there is only one reason that a true submissive cuckold submits to his Wife's pleasure: his love and adoration of her. (OK, so that's two reasons.)
However, this is also the cuckold's saving grace and ultimate strength, the thing that binds our Wives to us stronger than anything her lover of the moment can give her. A wise woman knows that no matter how exciting and passionate her sexual relationship may be with her lover, it is very likely temporary. Her cuckold husband will be there to love, adore and serve her always.
Her lover of the moment may not be very understanding of her desire to have sex with the new man she just met, but her cuckold husband will dutifully help her prepare for her date with him, wait anxiously, hoping she is having a wonderful time and that she will return home happy and satisfied to cuddle up with him and, perhaps, enjoy a little "night cap" love making with him as well.