Yup. She is unnerved for a reason.
As Paul Coelho wisely said: Everything that happens once can never happen again.
But everything that happens twice will surely happen a third time.
The very fact that the friends at the dinner were all complicit in their knowledge of her husband's bisexual past indicates that her husband went quite a bit beyond your usual one-time-and-never-again-dorm-sexploration with your drunk roommate. Sure, the whole m2m aspect of his bisexual past is all history now. But histories of this kind are never really forgotten.
The real disturbing issue here FOR HER, is that her husband has a possibly viable alternative to his married life with her. Should she choose to push her envelope too far, he may start thinking, and looking into other alternatives.
Hence, it is perfectly understandable that she is, putting it mildly, 'unnerved'. Her role as the owner of the monopoly of his happiness in life is now in serious doubt.
Putting it very plainly, she used to think that she was having the dude by his balls, and now the doubts are really showing up.
If the roles were reversed, the guy would be, most probably, far less unnerved.
To many, many men out there, sex without real life penis is really no sex at all. So, whatever the two (or more) women were doing together sexually is really of hardly any consequence at all. It may be (or not) stimulating to view, but it really doesn't matter.
On a daily life level, relatively few women are major family providers. So, if his wife were to choose to look for her happiness with another woman, this may or may not come as a shock to him, but since he is probably earning more than she is, he will be able to sustain his life without her anyway. And, sure, there are other women around, too. Moneywise, her choice is less likely to bring about his life to collapse than if the roles were reversed.