See subject. By mistress, I mean the definition “an illicit lover outside of wedlock, female.” Is there a male equivalent?
the closest I can figure for disappointment is “pleasant surprise” but it’s 2 words, and doesn’t really convey the meaning. I don’t suppose you can say “Wow, that movie was really appointing!” can you?
Cuckold?
Satisfaction/satisfying?
Well, in northern New Mexico, the term “penko” is used for a male extramarital lover. I don’t think it’s proper spanish.
Actually, I’m not sure that would work; I think that’s strictly a man whose wife is cheating on him. Maybe “cuckolder.”
Paramour is a word for an illicit lover, though it is not sex-specific.
The husband of a woman who had an illicit lover would be a cuckold, not the lover himself.
A slangier term for an illicit male lover is “back door man” (“the men don’t know, but the little girls, they understand”).
“Disappointed” means that there were expectations, and they were not met. “Pleasantly surprised” does not seem to be the opposite of that; the opposite would be that expectations were met, not that they were exceeded.
The antonym of disappointment is obviously “appointment”. It’s a perfectly cromulent word. 
I guess technically, the antonym of mistress is wife…
For the opposite of “disappointment,” I would suggest “serendipity,” that is, “the faculty or phenomenon of finding valuable or agreeable things not sought for.”
That is one interpretation, and in fact Merriam Webster gives the antonym of “disappoint” as “satisfy.” However, I find that unsatisfying. Another interpretation would be that if you are disappointed, what you found is less than what you expected; so that the opposite should indicate that you found more than you expected.
Elation?
doxy
Some words just don’t have antonyms. I’m sorry to have to tell you this.
toy boy?
From Merriam-Webster online:
Entry Word: disappointment
Function: noun
<…>
Near Antonyms bliss, felicity, gladness, happiness, joy
Antonyms contentment, gratification, satisfaction
I would suggest relief or fulfillment, as other near-antonyms of disappointment, depending on the context.
I would suggest inamorato as another word for a woman’s lover, whether she be married or not.
I wish some respondents would take this question more seriously. Y’know, be more chalant about it.
Mimbo? Gigolo? Piece?
I think “satisfaction” is the best antonym of “disappointment.”
Hmmm, by the same logic the antonym for “disappointment” would then be “appointment” 
The actual equivalent of mistress is giggolo.