The Amphur in Pathumwan is here you have the ability to register your marriage as well as register a divorce. The address for the Amphur Pathumwan has been added below. You can find this online as well. If you are going to get married then your will have to contact the Amphur or District Office in the area. Speak to a lawyer in Thailand for more information if it does become complicated. This as a prenuptial agreement has to be registered together with the marriage registration.
Note that you can register a marriage with a prenuptial agreement. You will also need to do this at the same time. The Amphur office will also allow your Thai wife to change to her marriage surname as it will require a name change certificate. Always take proper legal advice before you register your wedding in Thailand. Note you will need to have a letter of affirmation which has been certified and translated by the Department of Foreign affairs before you go to the amphurs office.
216/1 Chula Soi 7, Wang-Mai, Pathumwan, Bangkok 10330