My advice might not be the best, but I think you should not try to pursue a man like Jeff unless you are okay with an open relationship. If he is a "straight man whore," it's very unlikely that you two will ever have a happily ever after story. It sounds like he's sexually curious, but don't ever assume he's ready to be exclusively gay for you. If you already get jealous at him sleeping with other women, you'll never be happy with a man like that. Even if he stops being who he is (a straight man whore), do you think you would really be happy with yourself knowing that you are holding him back from being what he really is (straight, or if anything, bisexual)? The problem is that it seems like you are trying to fantasize that maybe you and Jeff will end up in a happy gay relationship, but with a bisexual or straight man, that's never the case.
I think that's happened to almost every gay man - falling in love with your straight best friend. It happened to me too. We've been friends since second grade, and we're still the best of friends after college. I've sort of always had a crush on him. There was a period where he would joke around about us being gay, and it would excite me and lead me to fantasize about the idea, but if you're really best friends with a person, you know that they're really not gay if that's the case...and with him, that was the case. I knew the feeling would never be reciprocated. I still like him. Unfortunately, I think I might love him, but I always try to just suspend those feelings in the air. The best we can do in cir
stances like these is just wish the best for those who are great enough for us to fall in love with. That's what I've done. He started dating a girl really early on in high school. At first, I was naturally jealous and I tried to steer him away from her, but as I got to know her a bit more, I realized that she's a great girl and that she'd definitely make him happy. I'm good friends with her, and I'm glad for him to have someone as great as her. Now, I've constantly done all that's within my grasp to keep them together when other people try to break them up or stir up controversy...even though I'm still madly in love with him. You really just have to come to terms to what's best for those you love. Sure, I can tell him that I've loved him maybe even subconsciously since we were children, but that wouldn't really help anything. It would, if anything, make things awkward. We would probably just say that everything would be fine and slowly drift apart. His friendship is more important than my own urge to have him the way she gets to. Sure, that doesn't mean I don't fantasize about it every now and then, but I've gone beyond thinking it's something that would ever work out. I have him now as a great friend; he's straight, so that's all I can ask for.
Don't try to get caught up in this whole "maybe I'm the exception" mumbo-jumbo. It's amateur gay fantasy at its best. Sure, maybe you can be his gay experimentation guinea pig, but if he has sex with women, chances are that his attraction to women will not evaporate suddenly into nothingness. If you're okay with being someone who is attracted to women or who might leave you for a woman, then go for it, but it sounds like you deserve someone that can give you more of a dedicated devotion. Jeff doesn't sound like that guy. Let him fly and let yourself fly higher - lingering to this feeling is just going to keep you trapped in the bird cage.
Moral of the story - if you really know and love Jeff, you'll do whatever you know is best for him. Unfortunately, that usually means suppressing your urge to have him. Let Jeff have someone that can truly make him happy. Even though your feelings for him will probably linger for a long time, just wait it out. One day, you'll find someone that's right for you - someone that's ready for you! You deserve much more than a man that might just probably be curious enough to explore an untouched region of his sexuality with you.
Regardless of whatever you do, I wish you the best. I hope you don't lose him as a friend.