Hope this isn't too off topic.

Problem: My two closest female friends keep trying to get me to show interest in them though I don't think they have any serious interest in me.

It offends me because, though I find value in their friendship, I can't stand that they want to imbalance the relationship in their favor. One of them is single (S) and one of them is in a LTR (R).

R has been in a LTR since before I knew her. I befriended her knowing this because I wasn't attracted to her (physically I mean) and she is really cool and smart. I am your textbook AFC (by which I mean I'm not very assertive or touchy with people), so she has deduced that I am a virgin (though we only see each other every few months now that we are no longer in class together and she has no idea) and drops snide remarks that are all in the vein of: "One day maybe you will discover women."/"I bet that went over your head!" (when something sexual is said). The only thing she can truly base this on is the fact that I have never hit on her. I find this really arrogant and annoying. If she wants to tell herself I am an immature virgin and that's why I friend-zoned her (even though she was never available) that's fine, but stop dragging me into it with insults.

S on the other hand befriended me. She was probably interested by the way she acted, and so was I at first, but being an AFC I couldn't pull the trigger years ago when I had the chance and it is probably for the best because she is pretty lonely and would make an extremely clingy and petty girlfriend. I don't know if she is still interested because it's really hard to tell with her, but I'm not and I've never hit on her either. Still she sporadically drops stories about how pretty some guys she met thought she was, clearly trying to get some reaction from me on the subject, but that is not how this works. If we're friends, we're platonic friends. I refuse to be a friend-zoned guy.

My question is how do I react to this? I feel like I'm being shit-tested. So just answering "okey", "sure" in a NGAF-way has worked fine, but they don't stop. I don't want to lose them as friends, but I might have to if I can't beat these tests once and for all. I have no patience for friends who want me to give them a free ego-massage. Also trustworthy friends are hard to come by.

Do I have to put up with this or can I "win"?