I assume that you two are still in your late teens to early twenties. In this case, generally, he should be single if he goes on a mission. Once he marries you, his responsibility is to take care of his family. That means earning a living. Few people can earn a living while serving a full-time mission. Now, if you two would be independently wealthy and not have to earn a living, then there is something else to keep in mind. Many missionaries leave girl-friends/fiances behind when they start their missions. I can't remember the exact statistics, but the vast majority of these women wind up not waiting for their missionaries. They get lonely, and move on to someone else. Of those women that do wait, many decide not to marry their missionaries. The couple just grows apart over the two years of the mission and find that they are no longer compatible. So, very few missionaries actually marry the girl they leave behind. Do you really want to subject a marriage to these kinds of stresses? Of course, if money is no concern, you might be able to get married and serve a mission together. However, if you are wealthy enough to do this it would probably be better for you two to take some time to get used to living with each other before you actually went on a mission. Additionally, if you two are as young as I think you are, then the church might not want risk the possibility of you getting pregnant while you are on you mission. (Having the responsibility of raising children can be very disruptive ones ability to fulfill a full-time mission.)