Okay, ill try and break this down for you a bit.
Sheffield has two locations where there is accomodation. The first is City & Central and the second is Endcliffe Village. You can find out all about these on the Sheffield Accomodation website so I suggest you have a look at the different types available.
Most of the stuff in Endcliffe is single sex at the moment but they are slowly moving towards more mixed flats. I'd say about 80% of it is single sex and a further 20 is mixed.
Trust me living single sex is not as bad as you'd think, I love my flat to bits and am kind of glad I just roll around in my PJ's and not worry. You end up getting close to lots of the single sex boy flats as well and so it all evens out in the end.
I personally think that Endcliffe has a better atmosphere, as its a very lively place. I know lots of people in City and Central regret not picking it to be fair.
If you have any more questions fire away!!