LDS = Latter Day Saint (also known as a Mormon)
I went to school with a guy who is Mormon. His religious beliefs weren't a big deal at all. His friends ripped on him for being Mormon in high school, but it was in a good kind of teasing way, not to be mean. He was pretty popular cause he was really funny. Like, for the costume contest on Halloween, he showed up to school dressed as a workbench. He had made a sandwich board type thing out of plywood, and he had actual tools attached to it - hammers and screwdrivers and drills and stuff! He said it was SO heavy, but he clunked around to every class in it... but then he'd have to take it off so he could sit down LOL. It was hilarious though.
Anyway... as far as a fic goes, I think it would depend on how the Boys reacted to dating someone with different religious beliefs than them. Personally, I would have no problem being in a relationship with someone from a different religious background than me, as long as they were a good person. I mean, there's religious beliefs, and then there's just basic morals. The morals are more important to me. Most religions have those in common, even if the spiritual beliefs may differ. I am pretty open-minded about religion though; I don't necessarily follow even my "own" religion to a T. Going with the LDS example, I may not agree with everything the Mormons believe, but I like a lot of their values. But maybe it would matter more to someone like Brian, who is way more hardcore religious than I am.