It's not really a matter of future vs. present. A sim can only have one "partner," defined as Steady BF/GF or higher up through spouse, as there is only one field in which to hold the partner sim data. But this field is world-specific, so a sim can indeed have multiple partners in different worlds. Oasis is only an example.
This only becomes an issue if both partners ever end up in the same world at the same time as is what happened here. The game will not (or should not) corrupt if this rule is broken, in fact there are several in-game ways to get what appears to be multiple partners in the same world such as the social networking skill relationship transmogrophier, I think maybe by using elixirs or other magical things, and certain photo booth scenarios, but typically this doesn't happen every day. The seemingly multiple partnerships though will not be stable and yes, can be quite a joy to try to play through.
However, what happens in Oasis (or Uni, France, Egypt, China, Traveler mod connected worlds) should stay in Oasis (etc.). :)