@Anniej5
I've had no problem with the actual breakup happening. I've never had a rejection. But how I do it may be the difference. You might be trying to do it a different way.
I basically have my sim max the romance bar with whichever one of the couple that has the right gender preference, usually making them a bf/gf/romantic partner and then use the convince to leave spouse option to break them up. And then I break up with them. Of course, breaking up gives you a max negative romance bar but doesn't seem to affect friendship as far as I can tell.
Hope this helps.