I've had the same thing happen to me and for what it's worth here are my thoughts.
Firstly, if it were me, (and once upon a time it was) and I was the only one seeing this young lady I would have kept that to myself, as somehow I'm sure she wouldn't want all of perb knowing about this.
Secondly, I know the pain, it's sort of like being kicked in the gut..it can actually physically hurt. That said, you do have to respect her wishes, and if you
want to stay in touch with her, I think that's great. I was out of touch with the lady I was involved with for almost a year, but we've been bantering back and forth with e-mails for awhile now and we're going out for lunch next week. As hard as it is, the break allows you to put the whole thing in a different, more detached perspective, and allows both of you to re-evaluate things.
If you've had the kind of relationship you've described, then there's a damn good chance that she values the relationship almost as much as you do, even if she don't value the sexual aspect of it, she must value the fact she can depend on you. That in itself has tremendous value. The hard part is that more to push for the status quo, the more she's likely to distance herself from you. At one put in my situation, I just gave up and stopped contacting her, and about 3-4 months later she contacted me. One thing to consider, if you really care about this woman, is that it's a marathon, not a sprint, as I've been involved with the above described individual for a little over 4 years, which is well beyond a fling.
As to the boyfriend issue, best just to let it play out the way it's going to. In the second year of our relationship she was actually engaged, while seeing me and even wanted me to try and get her boyfriend a job. Hell, she send me his resume. Her relationship with him only lasted a few months, but had it continued, I'm sure she would have put a stop to our get togethers.