True Life: I fuck my best friends who are girls

I've entered another relationship with one of my closest friends...again.
 
The first girl, Sara, I had been friends with since 7th grade, and I lost my virginity to her, banged a few times, and after we stopped messing around actually remained friends.
 
Kristen was a real cool girl, met her in college, blazed together, had the same taste of music [real big turn on] and were from the same area.  After a month of hanging out, ended up dating and having sex and stuff.  
 
   Sadly I'm no longer going to that college and Kristen cheated on me with my boy.  So I'm home now, chillin, hanging with that girl I mentioned Sara, and her other other friend Grace, who've I've always been particularly close too. These girls go to the local college, I took a semester off. These girls and I hang out with the same friend group and as I said, I fucked one of them already.  Well a little while ago I got hammered with them, party hopped 3 or 4 houses, went back to graces' place to crash while Sara went home and Grace ended up venting to me and really opening up.  I'm a nice guy, I supported her and she ended up kissing me.  We banged two days later and vowed to have an open relationship, but continue having sex with each other, as long as we don't ruin anything or get attached.
 
Grace is an awesome girl, admittedly she has big shoulders and is a little big, but were not talking huge, just broad shouldered.  She's been one of my closest friends for 2 years now and we've always had a weird relationship because of how tight we are and she actually helped me get with Sara.
 
It's a little taboo too because were not telling any of our friends and it's a little hot because we hang out and just smoke, like the last two times we chilled I just hugged her goodbye since shes on her period.  But were both down to just get down and I can talk about other girls in front of her.
 
I just want some thoughts on this, I can't ask my boys.  Told my bro and he said end it, but Idk, the sex was kinda fun and the way were going, were doin alright.
opinions? comments? jokes?  Let's hear em'

 

I'm Living it.  Say to her all the time, no regrets, no drama, all about a good time
 

 
Could def get attached, but I mean with sex how do you not?  I'm more worried of an over attachment.  Like I'll keep my shit under wraps,  but since we've been friends for so long, she might get really attached.

Multiple times
 

I feels man.  I like just being able to blaze and chill with her.  We vibe.  Idk what we are but I like it.  I've told her she can get with other dudes,  She's said I can get with other girls.  To kinda push the envelope last friday I actually did get with a girl right in front of her.  She got upset, but ended up alright and in the end I still went home with her. We've already come across some issues but were making this thing work.    
 

 
You read about my sticky situation

Idk Mister Samurai, I don't want a relationship right now, neither does she, we both like to fuck, we both are really tight.  I text her everyday, but even if we weren't fucking I'd still play the phone games.  When we end this thing, I don't think it will be on bad terms.