Personally, I would think JW or not anyone who:
was to have an affair with a married person knowing that they were married
is going to feel guilty and not so good about it if he/she has any kind of conscience or religious convictions (even if he/she is emotionally involved in the situation). She/he is going to have issues with it.
Being a JW just adds a whole nother degree of difficulty to it all. The person will not only have feelings of sadness and guilt but also be faced with losing her/his friends and possibly family (if they are JWs) if the fact that they did this is made public to other JWs.
He/she will either hide it from others or confess it and be thrown out of the religion. Granted, being disfellowshipped all depends on whether it is one instance of sex or a continuous affair, which has not ended. It also depends on the particular group of elders (priests) this person meets with to discuss the affair.
IMHO - for him/her to get to the point of having an affair with a married person he/she is already at the point where her/his spirituality is not a major concern and possibly he/she is looking for a way out of his/her religion. In that case, he/she may cling to the person with whom she/he is having the affair or may end it once they are free of what they were trying to get away from (being a JW).