Similar logic as to why would you lie about it being “a one time thing” will apply if you tell him now prior to him later finding out, in that if you were going to lie, why would you tell him at all. If he finds out later after you trick him into marriage (he will look at it this way), he will have good reason not to believe a word that you say, and he will never believe that it was only one time. He will view the whole the other man (OM) was just a friend thing as just a lie to allow you to spend time with your affair partner. If you are at all in contact with the OM after you cheated with the OM, your fiancй will believe that you did not tell about the cheating because you wanted to keep the OM in play.
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