You've got to remember that there is a huge difference between fantasy and reality. People ( men in particular ) can be arroused by a huge variety of different things, and people have fantasies about the most amazing and bizarre things which they would never dream of actually doing or experiencing in real life. This is normal and natural.
Studies show that even professedly 100% heterosexual guys are arroused to one degree or another by the sight of penises ( whether they're aware of it or not ) and erotic pleasure or sexual arrousal in this situation may have nothing to do with their sexuality. It's a conditioned response, perhaps related to the fact that studies also show that men often produce more sperm when they have sex with a woman ( or a man for that matter ) in the company of another man even if they are sexually turned off by the presence of the other man and have no general sexual interest in men. There is thought to be an evolutionary reason for this.
Now on top of this, as others have pointed out, it's perfectly normal to be as suddenly not arroused after you've cum by whatever fantasy was getting you off before you came, and you can see why you might be having this seesaw relationship with the idea of sucking a dick.
It's not like all people who masturbate to the idea of getting beaten with a baseball bat and pissed on by a crowd of thugs would ever actually want that to happen and they might just suddenly be turned off by the idea once they had relieved the sexual arrousal created by that fantasy in the first place.
However having said all that, some fantasies
do represent a genuine desire for an actual sexual activity to happen, and in lots of cases people's fantasies are an expression of real sexual urges. The trick is not interposing any psychological barriers between yourself and your fantasies ( unless we're talking about anything non-consensual acts that is LOL ). Saying that after you cum "there's no chance in hell [you'd] want anything to do with a cock of any kind - at all" sounds like you've decided already not to be open to the idea od actually exploring what your fantasies have suggested, which is a pretty strong psychological barrier. It may be a reflection of fact but it's worth keeping an open mind and not pre-deciding the outcomes of you entire sex life before you've given every option a fair chance.
In short, just because you get off thinking about big cocks in your mouth doesn't mean your definitely gay or bisexual or just sexually epicurean, but deciding your definitely not interested in actually exploring your fantasies before you've even explored might be denying you experiences you might find incredibly sexually fulfilling. Just keep an open mind, don't pre-judge, even if you never act on your fantasies it's better to be open to the opportunities life might throw your way than to completely deny yourself the prospect of ever taking advantage of them.
My sexual preferences change drastically from before I've jacked off to after. Before, I can get SO turned on by the thought of sucking a guy. I literally get the feeling that if there was a cock right in front of my face at that moment I would not hesitate to grab it and suck until his eye balls pop out. When I come, there's no chance in hell I would want to have anything to do with a cock of any kind - at all.
This is very problematic for me. I feel like I want to explore my sexuality and try stuff, but I am discouraged from doing it because I know that I will be 100 % (literally) turned off once I come.
Has anyone else experienced the same thing? Any advice?
Thanks.