There is no such thing as mixed = good, single sex is bad, just as you can't say single sex is better than mixed. Just as you can't say grammars are better than comps, or comps are better than faith schools. Generalities just don't work when it comes to schools.
Some single sex schools will be wrong for you child, just as some mixed will be wrong for another's child. People make their judgements by their own experiences and what they hear on the grapevine. If single sex schools were universally worse than mixed, they'd have been banned.
We worried about my son before he went to his boy's only school. We needn't have worried. He loves it there. Our neighbour's girls go to a single sex school - the elder loves it, the younger hates it.
Funny how a few people have said that the opposite sex are viewed as aliens by those who go to single sex schools. Funny because I went to a mixed and didn't mix at all with the opposite sex and yes, I did regard them as aliens and it was quite difficult to start relationships etc at first when I started to be interested. My experience was that the boys sat together and the girls sat together. Some teachers insisted upon it, i.e. girls in the left hand rows, boys in the right hand rows, but even in other lessons, it just seemed to be the norm that boys and girls sat on separate tables. It was only really into the sixth form where it became truly mixed.
If kids really want to "mingle" with the opposite sex, they'll find ways via out of school clubs/activities, and many single sex schools do have joint events such as school plays, joint choirs/orchestras, discos, etc - our son's boys school certainly do, and the boys and girls travel in together on the same buses so they can socialise on the bus too!
You have to look at your options and choose what's right for your child. Some people have a choice of faith/grammar/comp, others have a choice of single sex/mixed. Don;t let others influence you - your child needs to make an informed decision and that shouldn't be affected by other people imposing their positive/negative experiences on you!