I personally believe that would be very wrong and unfair..not only because I am a Christian and the bible says homosexuality is wrong but also because she is not the father and you don't know what could happen, you could break up and end up changing it anyway..thats my opinion, good luck and hope your pregnancy goes well
i say you give your baby your last name especially since all the others take after yours :) it would be less confusing for your children when they grow up. what if your relationship doesn't work out you will then regret your baby taking her last name.
think about it , hope you make your decision soon :) goodluck mamma!