Not too long ago, a couple folks blogged about taking kids into the field. In environmental consulting, you can't take kids into a lot of situations - you've got nasty polluted sites, and in a lot of instances, you need to have taken the 40-hour OSHA hazardous waste operator course to be on site. I can imagine some circumstances where it's ok, though, such as environmental restoration (where you're making the environment better for wildlife but you don't have a pollution problem per se). Pets can sometimes be ok to bring along, especially if you're out in the middle of nowhere.
Here's the problem: in environmental consulting, you tend to have a variety of field sites. Some may be close to the office, but a lot of times the fieldwork is beyond a daily commuting distance. You may have some flexibility in scheduling so that if you have commitments during the week, you can do local fieldwork or office work. But folks with kids tend to use up that flexibility. It makes sense, right? You never know when you'll have a daycare emergency or if your kid will get sick.
Meanwhile, there are all these other field projects that are too far away to commute to. In practice, the single and childless people do the lion's share of that work. That's fine, but I've run into situations where as a childless person, I end up never getting to work close to home. I've spent periods of over a year at a time going from one field site to another without a break. And if I'm working a 10-4 schedule and I'm flying back and forth (or driving several hours) on my "free" days, what I'm really doing is working a 12-2 schedule, with 10 of those days being 12-13 hours long.
After a while, it gets tough. I may have prescriptions I'm trying to fill, doctor/car/plumbing/whatever appointments I'm trying to schedule, and believe it or not, I may have the odd family issue that needs to be addressed. Just because I don't have kids doesn't mean I'm totally free of commitments. I got pretty pissed when a family member died (not immediate, so it didn't trigger official bereavement policies) and it was like pulling goddamn teeth to get a couple days off, while we had a decent number of people who apparently couldn't fill in because they had kids (not babies, and they had spouses who worked normal hours).
This brings me back to my previous post. If you're childless and have chronic family/health/financial problems and you'd like more flexible working conditions to deal with them, it can be somewhat hard to 'fess up (especially with problems that can have a certain stigma) and request more local/office-y work because that work is already taken. Maybe that's why folks who are having problems in the field are "protected" to such a degree by their coworkers.