I am Leader in the UK in an Explorer Scout Unit (ages 14 to 18). Our Unit has always had both girls and boys as members but our Scout Group's Scout Troop went mixed sex as well back in 2005. I can say that the way the Scout Troop does things has not changed due to having girls in the proceeding 12 years apart from we usually have separate tents for girls and boys to sleep in on camps and separate dormitories for each on indoor residential, oh and we put a bin in the toilet tents now. The UK Scout Association released some guidance/FAQs in around 2006 when they announced that all Sections would be going mixed sex which can be found at http://members.scout...insfin/omwt.pdf. The difference is that we went down the route of mixed sex Sections rather than separate boys and girls sections as it seems BSA are going for currently.
We have run camps with girls on it but no female Leaders on occasion and it has never caused an issue and in fact the last time this happened I paid it so little thought I never even noticed we were doing so until someone pointed it out after the event. In the unlikely event that a female Scout has their period and can't sort it out themselves then, as a modern man, I am perfectly happy to deal with that as its just another natural bodily function. We have a supply of generic pads which the girls know where to find which usually means we don't need to get involved any way.
Our Explorer Unit hasn't always had both female and male Leaders, and it doesn't cause an issue and no Explorer or any of their parents has ever raised this as an issue. I am more bothered about having enough good Leaders than what gender they happen to be.
As to dealing with relationships between Scouts, we generally ask them to keep public displays of affection within Scouting to a minimum and are clear with them that it would be inappropriate for them to share sleeping accommodation on Scouting events and trust them to stick to this and have never had any real issue with relationships in the 14 years I have been an Explorer Scout Leader.
Feel free to ask any questions you like and I will endeavour to answer them