Check about halfway down, a chart of calcium bioavailabilities.
The bioavailability of calcium in green vegetables largely depends on whether, and how much, it's cooked. Cooking breaks down phytic and oxalic acids, but you still don't get nearly all of the calcium out of them. Raw broccoli = very poor availability.
If you can't digest milk, there are other ways to get similar nutrition. Milk is a nice, convenient package for high quality protein and some carbs. Post-workout, if I couldn't drink milk, I'd probably replace the protein with a quality whey protein concentrate. Soy is another good source, but I'm not so sure about leaning on it for all my protein needs. Additionally, milk protein is actually a better protein, when talking human muscle protein synthesis.
In general, all animal products have things that leach calcium out. My wife is better read on the subject, but there are places in arctic climates where people's main source of calcium is fish bones, and those people have some of the highest rates of osteoperosis of anyone because the calcium is leeched out. But that might be from polution contained in the fish (fish can absorb lots of polution apparently), I'm not sure. Or maybe the whole thing is just something the vegans are pushing to discourage people from buying milk.
Pretty sure you're referring to the alkalizing/acidifying properties of different foods. as with most long-term health problems, osteoporisis is multifactorial. It's not as simple as saying "don't eat meat! It causes osteoporosis!" (not saying that was your tone, though). Like most things, it's balance.
Here's a review of the idea in the NYT. a prime quote: "Except for hard cheeses, which are acid-producing, most dairy foods, including milk, are “metabolized to compounds that are essentially neutral,” Dr. Dawson-Hughes said."
and
"They conclude that the high
fracture rate in countries that consume the most milk and dairy products results from the fact that “these affluent Western countries also consume the most meat, poultry and fish.”"
edit: note that the amounts of protein they recommend are laughable for those of us who are highly active. At 55 g of protein per day, I'd be at about 6 percent of my calories from protein. Not feasible, even if I tried. There are compromises that have to be made, and since I am about 2 standard deviations above the mean in bone density (and male), I'm not overly concerned with my daily consumption of about 1.6g of protein per kg of body weight. I also eat more fruits and vegetables than most Americans.
edit: you can do your own research on what is called Potential Renal Acid Load, which has been mostly studied due to its importance to those with renal dysfunction or gout.
This site has tables of values.