[how.we.operate.]

Personally, I feel that it is definitely crucial that you see if you can live together. If you don't get along while under one roof, you'll run into a lot of problems. This allows you to see what problems you have and how to learn to compromise.
Sex...I'm not so sure about this one. It seems like a lot of couples already have had sex before marriage. There are some couples that survive without having sex before marriage, but I think it depends on the couple and their sexual desires.
Definitely should meet the family. If your family doesn't like your boyfriend/girlfriend, there will be a lot of issues in the future.
I think one milestone should be finish college-this is if at least one person is going to attend college. Although it is cute to get married and there is a lot of pressure on engagement, I think it is important to get an education and focus on your education. I think it is alright to make plans/engagement before college ends, but a wedding should take place after graduation.
I think that the couple should date for at least a year. Marriage is a huge thing and you should spend a good portion dating, there is no rush to get married.
One important milestone that I'd say happens a lot, but it is good to recognize-work through a big fight/argument. If you haven't had a huge fight before marriage, how do you know how to work things out? Compromising and making up is a huge part of a relationship and it is vital to know that you can work things out.