Was Boromir a mistake?

Re: Comments made that the fellowship wouldn't have split at Amon Hen; Wasn't Aragorn already thinking about going to Gondor before the hobbits were captured? My understanding was that he was either going to do that or help Frodo to Mordor so there was still a very good chance that the Fellowship would have split.
If this has already been addressed, my bad....I didn't have time to read all the way through but I noticced several people saying that the Fellowship would not have split without Boromir etc. etc.