I asked my male sim to move in with my female but now he has her last name.....

Haha lol. That was a bit awkward. Anyways ideally if you want you're female sim to take the male sim's name then you get her to ask to move in to the sim house orto get married. You could correct your mistake though...if you still have the mans old house move him back in to his house for 5000 or move her back in for 5000 depending on situation and then get the female to ask her to move in..thereby getting her to move out of the females house into the male's house. However if you feel the female has an awesomer house than the male than you can do this complicated and expensive step. Move her into the males house for 5000 then once shes in his house move him in to her house for 5000 or more...then move her back in to his new house for like 5000 or maybe even more..that way she can keep her husband or her new wife's name and still live in her original house. But it's expensive..recommend you have 50K Simoleons for this operation.