[ on the ethics of drugging someone into "loving" you against their will ]

They say anything goes in love and war, or something like that.

Let's say that 'fake' love gave rise to a child. And that child is a happy child, and made both parents happy, cemented their love further, so to speak. What then?

You can't go back in time and undo it all, can ya? Even if ya thought it was unethical in the first place, breaking up what is now an idyllic family would be equally, if not even more, unethical.
But suppose, just suppose, you were one of the parents in that family, the one who used the potion specifically, would you now still be spending your nights wallowing in guilt? Or would you be focusing on the happy present, and let bygones be bygones?