"So, here's a semi-serious question. How would a non-christian (and non-white) guy go about dating or possibly marrying one of these Boise Mormons and Christians who take their faith seriously? Or in any rural Christian town?"
When it comes to LDS girls, especially ones in the smaller towns temple marriage is just about everything. So basically converting to Mormonism is about the only way. I don't really think that converting to Christianity is a big a deal with other mainstream Christian churches.
"I know Mormon girls usually don't have sex before marriage. Also, is it mandatory that the man convert to Mormonism before marriage? I've met a lot of hot Mormon girls and religion is not a deal-breaker for me. I also have a couple of Mormon friends but its certainly a big learning curve to convert. "
As stated above being married in the Temple is the aspiration so yes it would be absolutely mandatory to be join the LDS church.
"In these Mormon and Christian communities do they automatically shun any outsider who is not part of their religion? " Not shun per se but they will try and convert you pretty aggressively, otherwise you will be seen as an outsider.
"Do their parents and friends pressure girls to only marry fellow Mormons or Christians?" Absolutely, especially getting married to fellow Mormons
"Is a willingness to convert the only way these girls would ever consider marriage? " Yes for the most part.
"But these rural towns can be insular, so how will an outsider navigate the cultural and political differences in a multi-faith marriage or relationship?"
As much as I hate to say it, in the small towns 99% of it will come down to whether you convert to Mormonism or not, in a place like SLC probably not so much these days.
More about Temple marriage if you're interested:
https://www.lds.org/youth/article/why-te...e?lang=eng