Mr. Pink....I have went the friends with benefits route more than once. It's easy to get there, but really sucks in the end.
You Start a convo with her. Bring up that you "are a very sexual person ". Then you Say, "but gawd, there are so many diseases , among other things, who can I trust? Then...act like a light bulb just went off in your head and say... "I can trust you I know... I guess we both have NEEDS that need to be met, and since neither of us is with anyone...hmmm. Make her feel like you are doing her a favor. I know it sounds unattractive, but if she gets to thinking about the same things, diseases, a-holes etc... It will make sense to her completely. Don't stop there. KISS HER. Then Kino..LOTS of KINO. If she goes for that..guide her towards the bedroom for round 1 of many. If she gets mad or gets defensive...just laugh it off. Say something like..."I never realized you could bring that out in me, damn, where did that come from"! If she gets ma, I'd say cool it from there on out unless she makes a advance on you. If she goes for it, then you hit a homerun. Either way you got your azz covered!
Now for the really bad news. I sense you have a emotional attachment to her. Just remember all good things must end. What I am sayin is, that if she does go for the fwb route with you, eventually, she will find someone else she wants to be in a relationship with. Thats how it went down for me. the benefits will get scarcer and scarcer with her... then one day she won't give em up at all. If you have any brains, you will know why that is... someone else will be getting your benefits! Most likely it will be a new relationship she is in, and she will be fvcking him as much as he wants it. If at this point you have emotional attachments, it will feel like she is cheating on you, yet you won't have not so much as the right to say a word. cuz if ya do, she'll counter with "You and I are just friends" Or "don't you want me to be happy" Or "I knew fwb was a bad idea"! There will be nothing you can say or do about it really.
a sidenote, after all that, she will still want to be your friend probably. Then you get to see her parade around with her new Stud , and watch them make goo goo eyes at each other, etc, etc. How would you feel about that, knowing that when they leave to go home, you are heading home, and she is going home to give him head? Might be a little miserable for you knowing that! My opinion is if you are not at all in any way attached emotionally or in love even the slightest with her, might be a source for a few good times. If you have any feelings, you might want to sidestep this whole thing, and go out and get you a woman that can be all yours, in your own relationship.