I have a child at one and I have a few neighbours/friends whose sons & daughters go to LRGS and LGGS. None of whom have expressed any issues at all about them being single sex schools and in fact, most seem to think it's better! Lots of the older pupils have girl/boy friends from the other and they do a couple of "joint" musical and drama productions and a few older trips are joint (visits to tech and career exhibitions etc). There's also plenty of interaction on the bus to/from school.
The girls I know all say there's a lot less bitching because they're not competing for boy's attention, and the boys say there's not much messing about trying to impress the girls. Basically, they get on with the school work rather than trying to get off with each other!
We thought long and hard about choosing a secondary school, and yes, there are potential issues with same-sex, but at the end of the day, you have to weigh up all the pros and cons and choose what's best for your child given your choices. It's fairly easy to ensure your child has inter-action with the opposite sex outside school, such as sporting and social activities, to make up for the same sex school.
If there was a co-ed grammar in Lancaster, then, yes, if it had been as good, I'd imagine people would choose it in preference to single sex, but we've got what we've got, and if it's a choice of a top grammar single sex or good church schools or average comps, then single sex is a compromise I'm happy to make for a better education more suited to my child.