I don't disagree about the ADA, but it is illustrative of the point that it is not incumbent on the group being excluded, but the group doing the excluding to prove why that exclusion is appropriate.
I disagree. If we go back to before integration of blacks and later women into the workplace, they absolutely had to show that it was valuable to hire them, or else companies would have no motivation to accommodate the varying needs of their employees. Today we accept as fact that, for many jobs, people of both sexes and all races add value, but not all of them. A construction company which requires a lot of heavy lifting is probably not going to find female employees very valuable.
As for disabled people: companies didn't spend millions of dollars to provide handicap access just so that they could hire them as workers. Some built them because the law said so, others built them because they recognized that an image of being "friendly to the disabled" was good for business -- both from disabled customers and people who sympathized with them. I do agree with Random that the government forcing companies to cater to any group of people is contrary to a free market. You know what's worse than a government fine for not having a handicap ramp? A whole community of people who refuse to do business with you because the local news station just put out a report about how you hate the disabled.
Nevertheless, people in this thread have provided reasons why the exclusion is appropriate.
I also disagree with the argument that "it is the right thing to forcefully include women on submarines" because this isn't about sexual discrimination. Submarines, plain and simple, were not built to house two different sexes, just like an F-18 is not built to be piloted by men the size of Shaq. Aside from berthing, there are a lot of compartments that can't fit two people side by side. For men, it's pretty easy to shake off the fact that you'd be working that close to a woman, but what if you had to work in those conditions with a gay man? (insert cliche submarine gay jokes here)
So the solution would be to build submarines that can house both men and women, and convert ones that cannot. This would be a huge, expensive task to undertake. Additionally, at the current rate of building submarines, it would also take decades to fully finish the conversion.