To understand this post, you need to comprehend a couple things about Mormon culture. First, Mormons assign a very high value to Marriage. To us, marriage is an eternal thing. Once you are married on Earth by the proper authority, you and your spouse are sealed together forever, even after death. Not only is marriage eternal, it is also necessary for exaltation - in the life to come, you can't obtain the highest degree of happiness without a spouse. We therefore encourage young adults to not unnecessarily delay marriage. It is, in a real way, the "next step" in your progression. Because of this, Mormons often marry at younger ages than people in other cultures.
Second, Mormons are encouraged to serve ecclesiastical missions for the church. Young men are almost expected to do so - they typically go for a full two years at the age of 19. Young women, on the other hand, can choose to go if they want, but there is no real cultural expectation for them to do so. Unlike young men, young women go at the age of 21. I tend to think that guys go starting at an earlier age because it provides good training that keeps them out of trouble during those formative late-teen years. I feel like women are usually more mature at that age and are less in need of a rigid mission life-structure to keep them out of trouble... In any event, we serve missions because we feel it is important for everyone to understand the Gospel of Jesus Christ.
Now, to the point. It often happens in the strange world of Mormonism that young women who are dating seriously entertain the idea of going on a mission instead (or, more rarely, the young woman is willing to delay the mission to see if the relationship will pan out, but the guy involved thinks that the mission ought not be delayed). From a Mormon theological perspective, this makes no sense!
While it is important to spread the gospel, marriage is generally considered a trump card. Why? Because marriage is eternally important and, contrary to some commonly-held perceptions, the Lord can do His work without the aid of one more sister missionary. Of course, some will argue that "the field is white, already to harvest," and that there are souls waiting for the gospel, souls who "won't be reached if I don't serve." In response, I point out that I spent 80% of my proselyting time in more or less fruitless cold-contacting. If my own mission were really bursting with hungry souls, we wouldn't have been so actively engaged in searching for them and not finding them. If someone is genuinely prepared to hear the gospel, there are slews of church members and cold-calling missionaries who have time enough to teach them as soon as the Lord steers them in the right direction. Don't flatter yourself into thinking that there are people who you and only you can reach.
To be clear, no soul will rot in hell because you chose to get married instead of serving a mission. On the other hand, you might stymie your own progression by insisting that there is a dire need for your help as a full-time missionary. I'm sorry to break it to you, but... there isn't. This is why virtually every Bishop or Stake President you talk to will encourage you to pursue the relationship. Maybe you should take their advice.