Is AUTHOR wrong in the post ? POST : I'm sorry this will be a wall of text but will try to stick to only pertinent information. I tried to TL;DR, can't say it explains it all. 

I met my best friend (opposite sex for context, I'll call them Q) over 15 years ago.  

We slept together once in the first year or so of knowing each other. It was left at that and was never once spoken of again. 

Within this 15 years there are a couple in the middle we weren't in touch.  

Get back in touch eight years ago, we are spending a lot of time together, all platonic, I sleep with their best friend after all hanging out together one night within the first 6 months of us being in touch.  

We become friends with benefits for no longer than that summer, having hung out without my ever discussing it with best friend. (I was an asshole for this, I know)  

Fast forward to present, we see each other at least once a week, talk to each other more. We are both older now, single, and have become each others companion. Never argue, always great way of communicating with each other, hold no punches with anything ever talked about. We are each others confidantes, helping each other through some pretty shitty times.   

A couple of months ago, Q drops, "Maybe we should just get married." I question it, they shrug it off. Then they proceed to say these types of things many times in the following months.  

I sit Q down, we talk about, (about four weeks ago?) again won't tell me anything about how they really feel, shrugging it off again, but we continue to have a really crazy intimate talk about other things and we sleep together that night. (mistake #106 by now on my behalf, I know)  
  
Since then Q has again mentioned on at least three occasions, let's just get married. We just get along so well. They've has started dropping shit like this almost every time we see each other.  

So the last couple of weeks I started thinking about it. Maybe it's been there all along and I neglected to see it. So I decided I'd force the issue when I understood how I felt, which took us to Friday.  
We had a great time together, and went back to my place for a drink.  
Get drunk, sleep together again. When we woke up, I force the issue, this is either something or we stop the intimacy before it fucks up the friendship. We have two choices as I see it. We talk for a while and agree we both can see a future with each other. It took prying but it's finally admitted, they've pretty much had feelings for me for the entirety of the friendship.  
As soon as this happens, they mention that they knew about my sleeping with their best friend. The whole fucking time. All eight years.   
And Q doesn't know if they can see past it.  
Am I the asshole for having slept with the best friend? Because That's how it felt last night.   
Now in the light of day, I feel strung along and a little bit pissed off they've not been honest with me this entire time. And as soon as I tried to talk about any feelings we might have for each other, something I did 8 years ago is going to lose all opportunity to stay friends or anything more.   


TL;DR: Friend harboured feelings and when confonted with my own feelings it's thrown in my face I slept with their best friend almost a decade ago.

Please kind people of reddit, who's the asshole? I'm ok if it's me. I'm really just trying to make sense of it all. 