Can I switch from a 60-day tourist visa to a marriage visa in Thailand?

Well, you sure have been given some choices :O

I would say;

-come in on a tourist visa, get married <-by that I mean register the marriage at the amphur so it's legal),

-apply for a 90 day Non-Immigrant Type O visa based on marriage at your local immigration office,

-go back and get the 90 day visa and the new 90 day stamp in your passport,

-apply for a yearly extension based on marriage at the immigration office.

You will have to meet the financial requirements of 400K baht banked in an account in your name for 2 months or a letter from your consulate stating you get 40K baht a month income before you apply for the yearly extension of stay. But that will give you a year at a time here without the need to leave the country.

I know I am, even for citizenship. But due the fact Dutch Governments does not allow dual citizenship I will not apply.

It all depends on where you live. At some Immigration offices you can convert as mentioned your stay based on Tourist Visa into a Non O visa based on marriage, If not possible at the Iimmigration office near you than the easy way is to get a Non O based on marriage at a consulate or embassy. After that you can apply for Extension Of stay based on marriage.