Oh dear, you are quite in a predicament, aren't you? My heart reaches out to you; I seriously feel sorry for you.
Oh yeah, not that I'm insinuating anything or being extremely judgmental but are you a guy or a girl? You have a forum name that is not indicative of any gender, your profile gender is not set and, well, you don't have any pics either so I really have no idea...
In any case, I really feel sorry for you because I do think your best friend is a bit of a jerk. At the very least, you have an insensitive guy on your plate. I mean, what kind of sensible guy would just say that to a girl and not do anything about it? In other words, why should he tell you how he feels if he has no intention, or at least, no courage of his conviction to follow through and ask you out?
It is perfectly normal for best friends to feel attached to one another and when it really comes down to it, I really think he loves you (No, I'm not psychic; I'm just saying that if the two of you were really close friends, there is a high chance he has a place for you in his heart) If I might be audacious enough, it is easy to imagine that he loves you even more than that girl of his. However, it does seem that your best friend is being extra-cautious. Sorta like testing the waters; maybe if you said yes, he'd jump ship (ditch his girl and ask you out) I'm assuming you haven't told him how you feel. I know it looks greedy, and even rude, but, practically speaking, it is a wise decision on his part (only that it's beneficial to him alone) The heart, after all, is a very precious investment; you'd be better off knowing first what you're getting into.
It may look like he's keeping you as a reserve, or a safety net, just in case things don't work out with his girlfriend but have you ever considered that he might be keeping
her as the backup and that he's really after you?
Again, I'm not saying anything is real; I'm just saying that there are some things worth considering.
Okay, I know I'm also bing insensitive, thinking aloud when what you asked for is an advice, right? So on to the advice...
I strongly suggest you tell him how you feel. Tell him, with all honesty, that you are in love with him. Now, I know this looks like a very selfish move on your part for it will put him into hot water, appearing to make him choose between his girl and you but, if you look at it carefully, he started it. He most probably won't tell you he loves you out of pure honesty, hell, he's probably telling you that because he is hoping you feel the same way too!
Now, ideally, you can tell him that, even though you love him, you're not seeking to ruin his current relationship and that you wish for his happiness even though it does not include you. However, I would not want to ask you to do that if you don't really feel that way; it will come off as cheap trick and a quite too histrionic one too if you pull i off without meaning it.
Humanly speaking, the moment his girl find out about your confession, she'd be pissed off and there'd be a confrontation. Whatever you do, don't antagonize her or tell your best friend to ditch that slutty 3!+(# he's dating, despite any urge you may or may not feel.
Of course, things may or may not go as I have said but, whatever happens, stick to the truth. It was well that you didn't answer his question, rather than lie and say, "No, I'm not in love with you." Don't try any offhand techniques, cheap trick or dirty magic; it won't help the least bit and will most probably complicate the situation. If he asks for the truth, tell him the truth; don't lie. Tell him how you feel, that you are in love with him and that you feel like he's being selfish and that you think he's keeping you as a reserve broad.
From here on, you'll probably get a lot of other advices. (Well, duh, you asked for it
) Anyway, just to show that I'm not pulling this stuff from thin air, let me tell you a story...
I have gone through your... situation. Twice.
The first one was my best friend in high school. In our first few years in college, we grew closer together despite the fact that we're in different universities. Anyway, to cut the long cheese short, I admitted what I felt for him. Three months after, things went awry between us; we were expecting more from each other since we're already together and there was friction because, when it comes down to it, we were trying to change each other to suit our tastes perfectly.
We broke up and lost contact with each other for four years. When we finally met again, we were, at first, awkward but we rekindled our friendship gradually. We were, initially, just civil with each other, then warm, then we shared an open friendship. I know we can never be romantically involved with each other ever again but I'm glad I took that leap. More than ever, I think we gained a deeper understanding of each other. Now we know what we can ask/request and what we'd be better off keeping mum about. We lead different lives now and we rarely meet offline but we share an intimate camaraderie and a better degree of respect for each other than when we were just best friends.
The second guy was my senpai in my college days. It was quite an accident that I fell in love with him. Anyway, we were academically busy with school requirements that I vowed to myself never to say a word about it until after graduation; I don't want any possible friction between us to affect some projects we're working on together.
Well, things didn't go quite well on my part so he graduated and I, on the other hand, had to stay for another year. (As a side note, I didn't tell him that and put him off track with white lies, just so he won't worry about me) I confessed how I felt and, quite unfortunately, he didn't like me that way, at least, not as a romantic partner. He does respect my... preferences, just that he's not one of "us".
Things were awkward for the months that followed; we suddenly ran out of topics to IM to each other. I seriously thought that he feels uneasy around me so I started online conversations with him less frequently than before.
He got himself a girlfriend during that time, though he didn't tell me; I found it out from another person. Still, I know that trust is not something so easily given, especially after I have hidden some things from him all that time.
After some time, however, we soon fell back to the easy talk and light banter we often exchanged in our college days. Soon, we have regained mutual trust in each other. I trust him not to "out" me to our other (homophobic) friends and he trusts me not to, well, rape him or stuff.
Falling in love with your best friend is always tricky stuff but, as someone once told me before, it is better to say it and take the chance rather than keep silent and let it poison your friendship
v
Oh yeah, and about what BuffaloHELP-san said, I'd like to point out that you (bluefrostii, not BuffaloHELP) yourself didn't recognize it as well until he told you
Again, don't jump to conclusions and say he's keeping you as a spare tire. It could very well be the other way around
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You can almost sense some kind of psychological dependency on someone else from a far. And he choosing her over you says a lot about his ability to cast a good judgment. You don't need that juvenile as a boyfriend in your life. You deserve so much more! I don't know who you are but all of us deserve so much better. 
(didnt know how to change my profile,but i have fixed it.YAY
