Well, if he does go on his mission, dump his ass.
I am speaking from experience. Haha, no not my own experience. My sister's experience.
Well, I will tell you what I know from memory. This all happened back when I was in maybe grade 8-10ish.
Basically, back in the day my sister dated a Mormon. Apparently he was a real rebel back in the day (I was completely oblivious... possibly because rebellious Mormons probably aren't all that rebellious) and my sister cracked him into shape and made him a good boy. At least, this is my understanding and I could be completely wrong. He was awesome and was a real likable guy. I believe he may not have wanted to go on his mission originally, but my sister must have brought out the good in him and he ended up going.
My sister pretty much told him that if something happens and she finds someone else while he's away she would go for it. Because, let's face it. He's the high school sweetheart and he'll be away for two years. Plus, I think she probably was ready to break up with him soon regardless. My sister did end up finding someone else. When he found out, things got insanely nasty. Like crazy nasty.
Like all Mormons who go on their mission, they get slightly insane and extremely self-righteous. It happened to nearly every member of their family (all 9-14 of them).
Long story short, if he goes on his mission, do not wait for him. Preaching for two years can make Mormons go a little insane, and with you being an atheist, I just cannot foresee this ending well if he does end up wanting to fulfil the desires of his family and religion. It will probably take several months for him to get back to normal after his mission anyways. So you could be looking at not having a boyfriend for 2 years, and an annoying preachy boyfriend for another 6-12 months when he gets back.
Under normal circumstances, I would say if you like the guy, stick with him and you can probably learn to accept his faith or his beliefs. But, given the whole family's outlook towards you and the possibility that you will not see him for a very long period of time if he does go on the mission, I would recommend against sticking with him. Ultimately, he is going to have to choose between you or his family/church/community/Utah. If he shows interest in not following through with his obligation to his church, and feels that you are more important than making his family happy or gain acceptance from them, then by all means stick with him.
I didn't expect to form some sort of anti-Jessie brigade, but really, I cannot see this ending well. I would say accept him and his beliefs, but I cannot in good conscious knowing some of the several problems you could run into.
I'm just gonna stick around and see what happens. I owe him that much, eh?
...And since you posted this while I was typing away...
Yes. Perhaps see what happens. Don't end it right away. But don't set up any unrealistic expectations of this ending up happily.