So... I have a FWB. She's really hot, very smart, very nice and very well-liked, and there's a good 90% overlap in terms of sex stuff we like. Awesome, right? Well... I've known her for about 5 years but the more sexual parts of our relationship has started in the past few months. The main issue is, she's also my best friend. We've acknowledged that we don't want a romantic relationship. However, I've always had a thing for her and she's probably the greatest girl I've ever known. Really, really great person. So... I know it's casual but I don't want it to end. She's rated me a 7-7.5/10 and to me she's off the charts beautiful. So I really don't want it to end. However, I'm very afraid that some guy will come along and she'll cut it off. Not entirely, we'd go back to being friends, but I think I wouldn't like having to deal with it. She's amazing and yes, I for sure don't want a serious relationship at this point but I'd definitely feel used or offended if she just stopped being FWB for some other guy. Mostly because I'm not interested in any other girls I know for a long term relationship, (I'm still not out of school, so I'm pretty much stuck searching in that environment) and I don't have any close female friends like her. And I don't want to try and get one solely for sex. I don't like sex to be too casual, I do like intimacy and some level of caring for each other so she's perfect for that but I'm dreading the time when I'll have to give her up to someone she wants to be in a serious relationship with.

TL;DR: In a friends with benefits relationship with my best friend, but I'm moderately worried about the time when it will end, because she's not currently interested in me for a romantic partner and neither am I.