RETurkel wrote

<RYBS was very insistent on standing because a chassan (and kallah)  

And RJRich responded

<A  talmid of R' HS pointed me to Mpninei Harav (p217) where he states
the reason for standing given by R' YBS is the on the way to do a
mitzvah (bikkurim) reason.  The melech only applies once he's under 
the chuppah (and that's why R' YBS waited until then to put the ashes 
on the head of the chatan (that's when he becomes one)). but still 
stood when the chatan was walking down.>
 
     When I heard this reason from RHS, I questioned why, when in 
shul, we don't stand when a person comes in to put on t'fillin, since 
he is going to do a mitzvah; he had no answer at the time.  
Furthermore, if we accept this as the reason for standing, it would 
seem to apply to the chasan only, since the mitzvah is his alone.

EMT