[Age of Ignorance]

In March this year, I met a guy on an online game. We quickly became pretty good friends, playing this game almost exclusively with one another, and we started talking outside the game. We added each other on Facebook, exchanged numbers, etc. Our friendship is now pretty strong, and we have a lot in common... including his girlfriend.

In June, I met his girlfriend in-game. They have been seeing each other for 3 years. They obviously have a physical relationship. He spends weekends with her. Initially, when her and I met, I was keen to get to know her as a person, but didn't think much of it. We took her under our wing and we all progressed through the game together. Over the next few months, we would play at least a couple of hundred hours together, talking, playing, whatever. She always seemed pretty excited to have me around.

About 2 months ago, she sent me a message on Facebook. We started talking casually. We talked about a lot of things. We talked about studying, relationships, sex, anything. It was casual, no side-motives other than good conversation. We talked for a week or so, then things suddenly died down. We didn't talk that way for a while - we were both busy with work, and our conversations consisted mostly of in-game chat.

About 3 weeks ago, she sent me a message telling me that she was having dreams about me and wanted them to stop. She hoped telling me would get them to stop. She told me that they were sexual, in a way, and she's not sure how to get them to stop. This was the moment the flirting started. We may have been flirting casually before, but suddenly the flirting started building. It started slow, with quips about dreams and flirtatious patronizing. There was a back and forth, "enjoy your dreams about me."
Then it didn't stop. It intensified. Really quickly. We found we were compatible on a lot of levels. We talked until 5 am every night for a week straight (despite me having work at 9 am and having to get up at 6am). We talked about a lot of things. We flirted. They were generally great conversations, and I enjoyed them. We talked on the phone, through Facebook, anything.

Then she tried to tell me something at the beginning of last week. She tried, but she told me she couldn't say it. So we ignored it and talked about something else. The next night, in a very, very deep conversation, she asked me what I thought "about us". I told her we had a "thing" (exact words). She agreed, told me she was happy and relieved to hear it. Then she told me "you know I wouldn't cheat on my boyfriend though, right?" I read this entirely wrong. Completely wrong. I said it was fine, never expected her to, I was talking to her because I enjoyed the conversation. Then she questioned me a little bit harder about whether I saw "us" as a potential couple. I said I didn't, despite the fact that I did. She told me that we weren't on the same level, because she did see us as a couple... or at least, what could be a couple. I then went into damage control, and it ended generally emotionally.
The infatuation came on after I opened up over the next few days. Typically, I have this emotional wall that prevents me from saying stupid things to the wrong people. She pulled that down. Somehow. We were talking, had another deep conversation the very next night, and my mind just melted. Potentially from lack of sleep, or maybe it was me trying to salvage anything I could.

Anyway, things have built up. A lot. Everything has intensified. During an emotional conversation today, she asked me if it was possible to "love two people". This wasn't the first time she used that word. We still talk constantly. Her boyfriend stayed at her house for 4 days and she was talking to me the entire time. She told me she misses me. She has told me a lot of things. All of which indicates she's reciprocating.

But we all know how this ends. Someone gets hurt. It will likely be me. I get it, I know the drill. I've been in this position before. So I've mentally prepared myself (but I haven't mentally prepared myself) for the probability. I don't know what to do. It's terrifying... but as long as we're on the same level, I want this. Is that stupid? Naive? Idiotic? Most likely.
What do you think I should do from here?

tl;dr: I'm infatuated by my friend's girlfriend of three years. She tells me she loves both of us, and I'm willing to get hurt, but I want guidance about the next step.