I thought that in the episode they said it was because the whole nature of being a Trill was about new experiences, new people.. I could get if it was about leaving the new host to their own experiences and not involving them in previous loves, but don't think it was about this. Besides, the host/slug thingie are one new whole so should be able to decide together what will happen next.
The reason given for the taboo was that reassosiation prevents the symbiote form moving on, a really weak reason imho that also implies that the host is the one at fault when in reality the desire to reassociate lies purely with the symbiote.
A much better and imho more obvious reason for the taboo, one that would justify the harsh punishment involved, would be the fact that the symbionts past memories and desires may be unfairly influencing the thoughts and feelings of the current host to the extent that they become overwhelmed by it all and behave in ways they normally wouldnt.
The perfect example of this is the one night stand that occurs between Ezri and Worf in the final season. Ezri likes Worf and may even find him attractive but she has no true romantic interest in him. Their sleeping together came about because of Jadzia’s memories and emotions influencing Ezri, which is not too surprising when you consider that in the early half of that season Ezri spends a lot of time being overwhelmed by the memories and personalities of the past hosts to the point that she cant even remember what her favourite beverage is!
In Rejoined it is imho strongly implied that this is what is happening with Lenara and Dax. There is a scene where they are working together on the Defiant, the first time they have been left alone together, where Lenara explicitly states that she has never been affected by the emotions and desires of a past host so strongly before now. She then proceeds to get confused over whether she is Lenara or Nilani.
If the symbionts were not involved, if Lenara and Jadzia were just two unjoined Trill, would they have harboured any romantic feelings for one another? They would certainly have been friends and maybe that friendship might have developed into something more over time, but without the symbionts they would have been strangers who had only just met and Lenara was only assigned to DS9 for a short time so its very unlikely they would have formed any deep and long lasting romantic attachment to one another.
(also i dont buy the whole symbiont and host are one conciousness thing. Sure thats the explanation given but imho from the way Jadzia and Ezri behaved it came accross as host and symbiont being two partially joined conciousnesses performing a balancing act of sorts)