Do I need to register my marriage in Thailand to obtain a 90-day Non-O visa?

Reading the OP's question again - and in the light of no further comments from him - I take it he is not yet married. On that basis - no you cannot obtain a 90 day visa based on marriage until you are actually married. You can enter visa exempt, get married and then apply for a Non O + 12 month extension in country but the process for that is far easier if you apply for your initial 90 Day Non O in your home country. If you decide to apply in Thailand - others can guide you, I considered it but didn't do it in the end following advice here.

Douglas, can you please let us know if you are married yet or not? If you are married, did you get married in Thailand or elsewhere?

If you have a marriage license you can get the Visa, but you may find getting the marriage registered in Thailand to be very difficult. Especially if you don't have certification of your license in England with the UK government and the Thai Embassy there...

To the OP and anyone planning on getting married in Thailand. The marriage procedure changed around 18 months ago and it can now take over a month to complete the process. Apart from the usual Affirmation of Freedom to Marry, translation thereof and legalisation by the MFA, most Amphurs will not register a marriage 'while you wait' as they used to. They claim they have to 'check your documents' so you hand them in and come back later to actually register the marriage. Some Amphurs are quoting a month to carry out their 'checks'.

This can be a major problem to anyone who was not planning on staying long or is ‘visa time limited’. You need to get your partner to check this out with the Amphur you plan on using to register your marriage. Don't think you can change the Amphur later because there are limitations - you have to state the month and province you will get married in and if you change either, you have to go through the whole process again.

Apparently some Bangkok Amphurs can do their 'checks' in a day for a fee. The agency that translates your Affirmation will know which ones can offer that service (providing that service is still available) - again get your partner to call them and check.

So the process is............. book the appointment for your Affirmation at your embassy in Bangkok (if they require an appointment) then book your flight accordingly. Get the Affirmation translated and taken to the MFA to be legalised - an appointment is also required for that and it takes 3 - 5 days. I believe you book it online, not sure as I paid the translation agency a little extra do do that and I recommend anyone else does that. I've done the MFA bit myself before and its a real pain - especially as you have to go twice. Once you have your Affirmation and the legalised translation, you can take your documents to the Amphur of your choice and wait for them to do their checking. I also heard recently that one Amphur insisted on having a copy of the blokes Birth Certificate - never heard of that before and it really should not be needed - your passport is proof of your DOB. As its Thailand though, good idea to have it with you.

Well are you married or not, when I applied in 2020 had to show our NZ marriage certificate to Thai Embassy to get Non O for marriage. Then of course the real.paperwork war starts when in Thailand and getting extension.

I had to provide our marriage certificate when I applied for the Non O

You can’t get a “marriage” visa if you are not yet married.