Can I marry my girlfriend traveling to the US on a tourist visa after 90 days?

OK, so the answer is maybe. The reason is because you cannot enter the United States with concealed intent. If you have a visitor visa, then you can only use that to visit and not to remain permanently. However, there is no problem with changing your mind. If she intends to visit, but later, when she arrives, she decides that she wants to get married and remain here permanently, there's nothing wrong with that. For example, some of the evidence that USCIS and border patrol looks to to discern intent is whether she has a return air ticket. This is evidence that she only intends to visit. If they are suspicious when they talk to her at the border, and she tells them that she has a serious boyfriend, they will look at her phone to see what she is planning. If they see any text information on there that she decides to stay here permanently, they will send her back right away. So that being said, govern yourself accordingly. Consider working with an attorney. Some of us charge a very affordable flat fee to handle these types of cases and counsel can represent couples anywhere in the world.