It's actually quite common but guys tend to either not know, supress it or keep it a secret for various reasons.
It tends to reveal itself when a wife confesses to cheating and he finds himself with a raging hard-on.
I'm curious about how many couples are open about this.
There are usually clues such as the types of porn he watches, if he likes to watch you with a dildo (usually one bigger than his own dick), if he likes you to dress revealingly around other men etc.
You can usually test him by telling him about a "dream" or past experience you had where another guy gave you a mindblowing experience, he might protest verbally but his dick might say otherwise.