@Brandon ***********
This is not accurate. A girlfriend, partner, or wife can get a tourist visa. It is more challenging to demonstrate sufficient ties to Thailand if that particular American they are with does not also have sufficient ties to Thailand, so that is a thing... But it is not a hard no if there is a strong enough case that they will return. My wife (then unemployed girlfriend) was able to get a 10 year B2 visa on her second attempt. I know of at least three other Thai women in similar situations as her that also got the B2 on the first or second try. As long as it is not obviously a backdoor way to get her in to the country to change status or try to work illegally, you have a decent shot at an approval, all other things being equal.
There is no magic formula for approval. No amount of money in the bank or property or job type that makes it a slam dunk. Just provide the facts and make a compelling case.
It helps a lot if the American partner has a significant job in Thailand or is on a Thai Elite visa or has some other clear motivation for being in Thailand full time. I am sure there are many members in this community that were successful and would happily share some personal experiences or best practices... anyways, it is possible. The reported approval rate for B2 visas out of Thailand averages somewhere in the range of 70-75%. Anecdotal evidence not withstanding, Thai people get visas from the US (and Europe, and the UK, and Australia)... It might just take a couple of tries to dial in the application and find the right IO. Anyways, good luck to the OP... it only costs 185 USD to give it a shot... :)