I did wonder if it was possible for William to have got back from Australia to Scotland in time to marry, not sure how long it took in those days. 1869 he was a steward on a sailing ship.
Here’s a ship that took 79 days from the Lizard (often considered to be the last sighting of England for many in the 19th century) to Sydney, in 1868. I haven’t looked for shipping out of Glasgow or other Scottish ports.
http://trove.nla.gov.au/ndp/del/article/28422646 SMH 1 May 1868.
There’s plenty of shipping arrivals giving the number of days for the voyage from various UK ports noted in the newspapers that are freely available online at the following link.
http://trove.nla.gov.au/ndp/del/home You can determine names of the ships arriving in 1868 (to help limit the search options at the Trove link, not just to 1868, but to names of vessels too) by using the mariners site and entering ‘1868’ .
The Suez Canal did not open until November 1869, so the voyages from England to New South Wales perhaps averaged around ten – twelve weeks or thereabouts depending if sailing under wind or steam power, or a combination of these. Clippers perhaps made it in about eight weeks. Once in Sydney, there could be several weeks of unloading, obtaining new cargo, loading it, and passengers before setting out on the return voyage, back to England and perhaps intermediate ports along the way.
Marriage May 1868 in the UK, with baby born in UK in March 1869, seems to have your William still in the UK in July 1868
So earliest possible sighting in NSW would be say September 1868, assuming he had already secured a crew position on a ship out of Scotland ... And the mariners website seems to have listed most/all of the overseas arrivals into NSW for 1868.
Cheers, JM