Yes, it's completely normal. Reading fanfiction is a popular pastime for many people regardless of their appearance or gender. Twilight has a huge fan base, and fanfiction allows fans to delve deeper into the story and characters they love.
Well, it could be abnormal. A straight guy is usually attracted to the opposite sex. Enjoying gay - related sexual stories might indicate that he has some complex feelings towards sexuality that he hasn't fully explored or understood. Maybe he's just curious about different sexual experiences in a general sense, but it's still quite an unusual situation.
It's not entirely unheard of. Sometimes, in the realm of sexual fantasies, straight guys might be intrigued or turned on by things that seem outside the norm of their typical attractions. It could be due to the way the story is crafted, perhaps the power dynamics or the sense of novelty it presents. But generally, their overall orientation remains straight.
A normal plot could be that the couple runs a small business. The younger guy, who is looking for a part - time job, applies there. The couple hires him. Over time, they teach him the ropes of the business, and he becomes an important part of their team. He brings in new ideas, and they all work together to expand the business.
Definitely not normal. Normal social interactions do not include sharing erotic stories. It shows a lack of respect for boundaries and proper social norms. It can also make the person on the receiving end feel harassed or uncomfortable.