if you dont mind, please share in detail how exter marital affair started, how long did it take to turn serious, what avenue do you have?
personally, what we think right about marriage and married life in general is what television showed us/parents in the 80s, and before.
example of before television marriage-
i asked my 94 years old relative how his parents got married?
he said in those days, guy would just ask - "आऊं छेस्?" and if girl say "हुन्छ" then bingo, married, sex,
it didnt matter if he had wife in the house; he could bring another wife, and it was totally acceptable.
but then television portrayed a different happy family and substracted / relegated extramarital affairs as sin.
there has been a change/acceptable; you dont have to marry to have sex anymore.
so my friend, the internet generation is coming and people like you and others including highly respected members of sociery, will see a different society where extra maritial affair will be as common as couple having sex before marriage.
to answer your question, please fake it till you make it. accept that it is ok for her to have extra martial affair as well, as long as she can hide it. try to fix it; or ten years later, you'll probably not think that extramaritial thing is not really that serious.m and laugh about your today's situation