"I Do"

I recently noticed that at ~1:28 in "I Do," the tubular bells start playing in this rhythm:

I'm not sure of the technical differences (since one's in 4/4 and the other in 2/4), but this sounds similar to the rhythm in the Bridal Chorus in Richard Wagner's Lohengrin:

notation found here

I'm not at all familiar with Lohengrin, but it seems that this tune is often used in wedding processions, so it's fitting that the Beach Boys subtly reference it (or at least its rhythm) in a song about marriage.

"I Do"

I just learned the chords and most of the tubular bells for "I Do," which is included as a bonus track on the Surfer Girl/Shut Down Volume 2 re-issue.  When I listened to the album last week, I noticed that the melody for the verses of "I Do" is more or less the same as the melody for the verses of "County Fair" from Surfin' Safari.  The chords are slightly different though, which makes it a bit more interesting musically.

I'm not sure if there are strummed chords in "County Fair," but the bass arpeggiates the standard I vi IV V progression in G major.  Each chord last two measures:

I | I | vi | vi | IV | IV | V | V

The progression during the verses of "I Do" (in Ab major) is only slightly different:

I | I | vi | vi | IV | ii | V | V

The liner notes for Surfer Girl/Shut Down Volume 2 don't note the resemblance between the melodies in "County Fair" and "I Do," only that "Brian produced a version of 'I Do' for the Castells" and that "it was on non-Beach Boys productions like this that Brian really liked to experiment with new sounds."  In this instance, it seems that he also experimented with altering the chord progression under an existing melody.