Sounds like you are a voyeur, someone who gets sexual enjoyment from watching others have sex. And the fact that it's your wife having sex with someone Not You probably adds some extra oomph. It's a really common fetish.
Your wife may be an exhibitionist to one degree or another if as you watch her have sex with another man, she gets a sexual charge because you are watching her have sex with someone else. She could be getting off on having sex in a more public setting, breaking the sex=totally private taboo. Like many things, it depends on her mental state as she has this experience.
Personally, eh. I find watching others have sex that I am not involved in, rather dull. I am indifferent if others watch me having sex - there is no oomph for me to have people watch. So I am neither a voyeur nor an exhibitionist.
Also, I agree with CDM, dudes should be able to weigh in on this too. There are lots of people (straight, queer, trans) folks who may have some cool insights into voyeurism/exhibitionism for you.