The Gemara discusses a case in which a Jewish man marries a non Jewish woman with the tnai that the nisuin takes effect when she converts. Coversely, a non Jewish man takes a Jewish woman with the stipulation that it take affect when he converts. How can Chaim Yankel marry Mary Catherine with that stipulation? She is not currrently in the parsha. How can John Patrick marry Rivka Beyla? He's not currently in the parsha. Also - how can someone give a married woman a ring and say harey at mekudeshet li when your husband dies? She's a married woman now. She's asura to all other men.
These issues seem to be more complicated than whether you say you can make a tnai on something is ba l'olom or not or it should take effect when it does come be.
Of course, this Mishnah follows the opinion of Rebbi Akiva and Rebbi Meir that a person can make a transaction on something which is not yet in the world. In fact I would have said that it is easier to understand than, for instance the case at the top of 93a where the wife is Makdish the work she will make, because there, if for instance she is Makdish an item of clothing, that clothing is not in the world at all but even so Rebbi Akiva says one can make a Kinyan on it. In the case where the Jewish man does the kidushin on tnai with the non-Jewish woman, the woman is at least in the world, but all that is missing is her Jewishness. It is possible for her to bring this Jewishness into the world, i.e. by converting, and the Kidushin only starts after she converts. So even though she is not currently in the Parshah she is capable of bringing herself into the Parshah. Similarly the married woman is not in the Parshah but she is at least in the world and if her husband dies (and don't forget her husband might be seriously ill now) she will also be in the Parshah.