Depends what country you are from as you need firstly to prove you are single by getting documents from your embassy stating that .A Certificate of No Impairment. Try using Google to see what your Embassy can provide by looking up Getting Married in Thailand adding your nationality .
Been using Agoda but lately have been choosing free cancellation bookings in case of any dramas .Have twice had to cancel bookings and have been refunded.
I am only the messenger of this .If you don't like it please stay home .I was in Thailand in December for 5 weeks under the rules that applied then .I am going again late April and will follow their rules as they apply and make other choices to protect myself.