One Size Fits All...Or Does It?

Something I've been thinking about lately is how some Church teachings, principles, or commandments can be so "one-sized." That is, how one thing (be it a mission, getting married, having kids, etc.) can be (and in some cases, must be) for everyone. In a church that has over 12 million members, it's amazing to me that there are some things that are supposed to be universal. I guess I just don't really understand how a mission can truly be the best thing for all young men. Or why everyone must be married to enter the highest tier of the Celestial kingdom. Out of 12 million people, doesn't there have to exist one person who really doesn't fit the mold of being a spouse or doesn't even want (gasp!) to get married? Or, out of the 12 million people, doesn't there have to exist at least one couple who would be happier without kids?

I'm sure it's obvious from this post on which side my own biases are. As I didn't serve a mission, I can't really comment on that. However, while I really am enjoying being married, I still sometimes miss being single, and often don't understand why being married is essential to salvation. And on a scale of 1 to 100, 100 denoting an incredibly strong desire for motherhood and 1 denoting the opposite, I'm probably around a 7 (although that used to be a 4, so I'm getting closer).

So then, what do people do who do not necessarily want these things? Do you jump in and do it and chance unhappiness in an unfulfilling marriage and grueling demands of children? Or do you not jump in and therefore risk your eternal salvation?