I totally agree that the lead often knows who he wants before we do. And I imagine there is a lot of prompting and editing to make it look like they're still torn at the end -- I think TPTB ask some fairly pointed questions to get the footage they need. Something like, "so do you think you could marry X if Y weren't in the picture?" To which he could say, "I could absolutely marry X... she's amazing, blah...blah...blah..."
The downside of their needing to do that for the sake of drama is that the outcome seems a little less believable to the viewer -- we're left wondering how they could be so sure of their choice only a few minutes after talking up both women. And that also no doubt creates a problem with the F1 at times, when the show airs. That would be so difficult to watch -- especially if you're not watching it together!
I've often wondered whether any of the leads have ever actually confessed during the FS date that they've decided he or she is the one.
But I really do think Sean knows who he wants and I think the rest is mostly careful editing.