This may benefit from some context:
- Civil Partnership was permitted in the UK from 2004.
At that time, it was felt by many gay community leaders that this was a great breakthrough and a legal equivalence to marriage represented equality for gay people in the eyes of the law. Until 2010, gay rights campaigners Stonewall UK declined to campaign for gay marriage.
Civil partnership was seen as a good way to introduce legal equality without wrangling with the church over the use of the word 'marriage'.
This seems to be what Elton John is saying: civil partnership is a great step, and legally what's needed without also dealing with the issue of the word "marriage" at the same time.
However, over time, the gay community made it clear that they felt that this equality-with-a-different-name is not real equality.
- Gay marriage has now been legal in the UK for the last three days.
Elton John was one of the first public names to announce his intention to marry, and the article linked is about his advocacy.
It's not really possible from this quote to say that Elton John was particularly against marriage, even if he didn't personally want to get married at that time. I respectfully suggest that it was the Huffington Post trying to create a story.
On the point about a person's beliefs changing over time, Brendan Eich is free to make a statement to clarify his reason for making a public donation and update us on his current beliefs, if he chooses. His previous statement on this issue expressed no sense of regret.