The U.S. Presidential elections are not "rigged." There are more than 9,000 election districts spread over 50 states, it would be impossible for any group to compromise a majority of the districts needed to control the result of the election at either the state or federal levels. The loser should just accept the will of the people.
Proven examples of voting fraud in the United States presidential elections are extremely rare. Voter impersonation is fraught with difficulties and very rare, as is double voting. With so many safeguards in place to prevent fraud, there appear to be no genuine reasons for a loser to not accept the results.
No, there is not a legitimate reason for a loser not to accept the results because our country was founded on a democratic system. There must be a peaceful transition of power, or the country will crumble. We must look at democracies that failed and realize most of them had protested elections.
There may be some reason to question the results of an election and to request a recount. The Gore-Bush election is a perfect example. However, as in that case, the process cannot be drawn out and eventually the loser has to accept the results regardless, for the good of the country.