I get that, but I never said it had to be set in stone or anything. More like how you could choose how to treat the girls in the OG and how it would affect certain scenes later on. We know we'll have dialogue choices in the remake, so letting the player decide which girl to pursue or flirt with could allow certain scenes or parts of scenes to unfold differently depending on your choices much like in the OG, that's all, while still keeping the main story beats the same. And, incidentally, allowing the beginnings of something to happen with Jess, for instance, doesn't prevent the possibility of him becoming involved with one of the other two in later parts of the series given that this is the only one she's likely to be in and the fact that their romantic moments (Gold Saucer date for Aerith and Lifestream dive/Highwind night for Tifa) don't even happen until well after Midgar anyway.
Midgar is much too early in either of their series story arcs for a complete development of that sort. Where then would you go in the later parts if he's already clearly involved with one of the two? Foundations can be laid for both girls in Part 1, but I don't know that it would progress further than that at such an early point in the series. Should SE pick one and have one fridged girl (as you say) and risk alienating half the fanbase by not allowing them the choice for themselves? Or should they leave it up to the player to decide and simply allow three options instead of two? And as for your comment about Biggs and Wedge, there tends to be a difference of view for such things when it happens to girls as opposed to guys. Girls tend to be seen as more vulnerable, especially in situations like that where they experience a tragic death. It's a societal thing, I think.