I believe you should marry your lover if you two really love each other, marrying someone so that they can stay in the states is wrong and illegal, although it has been done so many times now that seriously, I don't think immigration cares that much. If your lover cannot renew his/her visa, and you two feel like you two love each other, then you should consider that before having any children.
Marriage does NOT automatically confer citizenship, legal resident alien status, or give any other legal reason for a non-US citizen to stay in this country. Please, contact the United Stated INS. (That's Immigration & Naturalization Service.) If his visa is "up", he has to leave -- married to you or not!!