This happens if you have proceeded the Sadie path too far and then you get "exclusive" to another girl. So you have only this way to end the existing relationship of Sadie (or another girl).
This happens at any "getting exclusive" - this is ending all other existing relationships. SanchoMod is showing you this.
If other pathes are developed yet, the MC (as a good boy
) has to go to them telling end of the relationship at least.
As lipe said it´s easier to avoid developing relationships to all other girls except your special target.
But anyway Sadie is a special exception cuz your "childhood love" is sooooo happy only meeting you again, it´s hard to reject her from beginning