What happens to my 90-day stamp when I leave Thailand on a Non-O (Marriage) visa with a re-entry permit?

If you have a single entry Non O then it is “used” on that entry. If you leave on day 75 then the remaining days of stay are null and void. You’ve used the visa.

You could get a re-entry permit to allow you to re-enter for the remaining days allowed if you wanted to. [edit] Didn’t see the bit about the re-entry permit before answering. That only allows you to stay until your original entry stamp. You don’t get another 90 days on entry.

If you have a multiple entry Non O valid for a year (unlikely as very few embassies will issue that these days) then on every entry you’d get 90 days up until the validity date of the visa.