Do I need to provide a flight ticket when applying for a Non-O immigrant visa for 90 days in Thailand?

You should check with the Thai embassy in the country you apply or on the evisa portal, in Italy it is required.

I have a 90 day multi entry visa based on married to a Thai and did not require or was asked for an onward flight

I am not familiar with any non-O visas that are 60 days. The non-O that I know is 90 days, and does normally not require proof of onward travel, although some embassies do require it (perhaps failed to understand what that visa is for). The 60 day visa that I know is a tourist visa, and that one does normally require proof of onward travel.

The Non-O Visa is a 90 day entry stamp and being a long term Visa does not require proof of onward travel