Dude, trust me! The younger me is the poster boy for this... think of a kid that everyone called Ferris Bueller and there you go:) Every girl's best friend- BLLLUURRGGGHHH! (tried and failed to make the throw up sound there...) I could tell you the most epic story of being rejected my senior year by my best friend that I put up for homecoming queen, won, and then she started dating my best friend... or the one in college I hung out with for 2 years non-stop while she banged all of my friends and then one coked up evening just asked her why we couldn't be to the point of it just being pathetic that I couldn't let it go. DON'T DO IT... we never hung again after that because it was obvious that I was smitten and she just wasn't... I was validating her and that in and of itself is gnarly.
And, then once I got exactly what I wanted. 4 years into this girl, best of friends and one night we split a bottle of tequila, blow each other, fuck and fall asleep. That was the last night we ever really hung out besides a few dinners with other friends to save face. It was more gut wrenching than the other rejections I had received earlier in life in a way...
So, as someone that's both won my "best friend" over and someone that has been completely mowed over by her I can say: Don't Do it. People that say they fell in love with their best friend don't realize the dynamic that they were in with that person in the first place... that there was sexual attraction all along. It does not work and I'll have a civilized discussion with anyone that tries to prove otherwise and pick you apart like Christmas turkey off the bone. Wait for the girl that you can feel it with and you can undeniably tell that she feels it for you too... those are the best relationships out there. Trust me, I've had everything else in between, too and I always hold out for those.
Don't wreck a friendship for sex. Especially if you're inclined to loneliness... you'd be amazed sometimes how even though you need to catch that nut every now and again a lot of times you simply need someone that absolutely loves you to just be with and hold even though it doesn't lead to wick dippage...