Courtney, you can get married. However, if your fiance (illegal alien) came to USA illegally without inspection and is not protected by some older provisions in the INA (sec 245i), he will not be able to apply for a 'green card' through adjustment of status even if married to a US citizen. No, you can not ensure that he will not be deported if he is in USA illegally. You should consult a lawyer prior to filing any immigration paperwork or deciding what to do.
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