I believe in polyamory, so I might be the wrong person to be discussing this. In my experience, a FWB/NSA relationship will not work if you're not both more committed to non-monogamy than you are to each other, and/or not open to the idea of polyamory. Yes, ultimately if it goes on long enough, feelings will become involved. It's human nature. You begin to see your sex partner as a complete person. Oftentimes, you find you respect and admire that person. You care about their happiness and well-being. If you are the type of person who can have romantic/loving feelings for more than one person at a time, it can be beautiful, in my experience. If you're ultimately looking for a monogamous relationship, it's probably not gonna work long term with your FWB, unless that person's also looking for monogamy, and sees you as a potential candidate for that type of relationship. I believe the best way to keep a FWB relationship going is for both partners to be completely open and honest about things. I don't necessarily mean divulging the details of each encounter with someone else, but letting your FWB know where you're at as far as your specific relationship any given time, and letting them know where they stand with you at any given time, and for you both to acknowledge that those things are likely to change for either or both of you from time to time. If you meet someone else you really click with, you'll likely have less time to devote to your FWB. If she/he meets someone else, you'll likely see them less while that's going on. If monogamy's on the table, your FWB thing could end altogether. Looking at these types of relationships realistically often sucks (who doesn't want to be someone else's first choice?), but if you don't look at it realistically, someone's gonna get hurt/be disappointed/feel used. And realistically, if you are each other's first choice, wouldn't you already be in some type of non-FWB relationship anyway?