I want to get married in a different state with my boyfriend but I am wondering if it is only valid in the state we are going to get married at and if I should take his last name because we want to to marry in a different state and later have another wedding on the state I am in
Family Law Attorney
You can get married in another *country* and it will be valid here. Legally valid marriages, no matter where they were performed, are considered valid in California courts. (Make sure that you follow the laws of the state that you're getting married in, i.e. get a marriage license if they require it and most do.)
If you have another wedding, later on, then it won't be a wedding so much as a wedding celebration.
As for the name change, this is completely optional.
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As long as you get lawfully married (i.e., follow the rules of the jurisdiction where you get married, such as apply for a license, have a ceremony conducted by an authorized person, and then submit the signed license to the County Recorder's office), then no matter where you get married, it will be recognized in any state.
There is no law in CA requiring you to take the last name of your spouse.