it's illegal for anyone to take your passport except border/customs and your own country government... you can leave them a photo of your passport or a reasonable cash deposit.
there are agencies in thailand that will check out your gf online to see if she is entertaining, sleeping with, taking money from other men, etc. it's worth the investment before investing everything into marriage where you carry all the risks and she stands everything to gain.. many thai women playing many foreign men same time... but different.. check your girl before you go through with it..
I spent 3.5 years going around different countries in south east asia on tourist visa.. maybe check out some other countries and places in the countries before deciding to settle any one place.