If you are an immigrant coming to the United States for the purpose of getting married, then the question of whether you can use a visitor visa for U.S. entry (maybe a multiple-entry visa that you already have), or should first obtain some other type of visa, hinges on one important question:
Do you want to apply for a U.S. green card after you are married? We will address both "yes" and "no" answers below.
Legally, there is nothing wrong with getting married while you are in the U.S. as a visitor (on a B-2 visa), if you return home at the end of your permitted stay. But that doesn't mean this is a risk-free strategy. If you haven't yet applied for the visa, you may have an uphill battle convincing the officer at the U.S. consulate that all you want to do is get married and then leave. Too many people before you have used visitor visas as a way of entering the U.S., getting married, and then applying for adjustment of status (a green card), to avoid the longer process of applying for a fiance or marriage-based visa from overseas.
And then, even after you obtain a visitor visa, the border official who greets you upon U.S. entry may not believe your intentions, and may exercise "expedited removal" powers to deny you entry and send you home. That would put an order of removal on your record, preventing your return to the U.S. for several years.
Given these risks, the safest bet is to obtain a fiance visa (K-1) for travel to the United States. It takes longer, but your entry will be easier -- and if you change your mind and decide to settle in the U.S. after the marriage, you will have the legal right to apply for adjustment of status.
Using a B-2 visitor visa to enter the U.S. with the intention of getting married and applying for a green card is a form of visa fraud. The visitor visa is a nonimmigrant visa, whose proper use requires entering with the intention of returning home (or at least leaving the U.S.) by the date on the I-94 Arrival/Departure Record. This form of visa fraud can result in the immigrant being placed in removal proceedings from the U.S. rather than granted a green card.
If you come to the U.S. as a visitor and only later decide to get married, that's a different matter -- you will be allowed to submit an application for a green card using the procedure known as "adjustment of status," meaning you would not have to leave the U.S. during the whole application process.
But what are your chances of convincing U.S. Citizenship and Immigration Services (USCIS) that you weren't planning the wedding from the start? For starters, USCIS will presume that if your marriage took place within a very short time of entry to the U.S. -- typically 90 days -- the purpose of the entry was to get married. No matter what, it's best to wait this long before the wedding. But USCIS can, of course, look beyond the date of the marriage for other evidence of your intentions. And because you will have to submit personal documents showing that your marriage is bona fide as part of the green card application, this could trip you up. If, for example, you submit a letter from when the immigrant was still living abroad, saying, "Love you, can't wait till the big date in June!" you can probably forget about convincing USCIS that your intentions upon entry were not fraudulent.
Of course, if you entered the U.S. as a visitor and then met someone and decided to get married, you have a good chance of presenting a convincing case that no visa fraud was involved, and succeeding with an adjustment of status application. But if you are still trying to figure out what type of visa to get for U.S. entry, a fiance visa is your safest bet. For information on these visas, see "Who Is Eligible for a K-1 Fiance Visa?" Or, if you're already in the U.S. and have gotten married, but worried that you'll have trouble convincing USCIS about your innocent intentions upon entry, you might want to leave for your home country by the required date and apply for an immigrant visa through Consular Processing.