The Commons have just voted 400:175 in favour of the Marriage (Same Sex Couples) Bill (HC Bill 126), despite a rebellion by nearly half of all Conservative MPs.

It's now got to go through Committee Stage and the House of Lords, but The Independent claims it's almost certain to become law later this year, while the BBC News coverage has some background.

Of particular note (courtesy of Wikipedia's summary):

Part 2: Other Provisions Relating to Marriage and Civil Partnership

Clause 12 and Schedule 5 amend the Gender Recognition Act 2004 to enable existing marriages registered in England and Wales or outside the UK to continue where one or both parties change their legal gender and both parties wish to remain married. It also amends the Act to enable a civil partnership to continue where both parties change their gender simultaneously and wish to remain in their civil partnership.

Very interesting - especially explicit mention of how it affects a couple formed of a M2F and a F2M.

Puzzlingly, there's also this clause:

In divorce proceedings, adultery will only include sexual conduct between two persons of the opposite sex;