I went through just about the same thing for nine years. I would be drawn to cock, have a sexual interaction and then hate myself afterwards. You're first step should be to stop focusing on everyone else and focus on yourself.
I took time (finally) to have an honest discussion with myself about WHAT I actually was attracted to. You can't think about labels like gay or straight. Just identify what you are attracted to in each sex. BE HONEST with yourself and don't try to convince yourself you are just horny.
Regardless of what some people think you don't have to be all gay or all straight. What do you like about women, what do you like about men?
I understand what you are saying about being in a relationship with a man. I could never go past friendship with a guy. I also am not interested in putting my dick into them in any way, I don't like getting oral and I am NOT interested in giving a man anal sex. I do like giving head and I do like recieving anal sex. There will always be things about sex that you like and that you don't. Try to identify WHY you like what you like and listen to yourself. I used to let guys blow me because I felt it was part of "a lifestyle", there is no such thing. It your lifestyle, surround yourself with people who respect your limits and understand you.
Everyone is right on here too. No one likes a sexual interaction they weren't comfortable with. I felt the worst back in the day when I got with some random guy. Spend some time and understand the personality behind the cock. You don't have to fal in love, but you still should know the guy a little bit. Always have an idea of what you are getting into. I got with guys that were pushy, were completely withdrawn and that seemed like they were just giant balls of lust. Always made me feel horrible. Set criteria: I don't hook up with guys who just write "I want to suck your cock.", I like 7 inches and more, the guy has to sound confident in what he wants.
Part of the bad feeling is probably because you feel like you're whoring yourself to random guys. That's why I felt bad. If you set criteria, you will feel a little better about the situation, you won't have to fight two moral battles at once. Right now you are probably fighting the moral battle of feeling like a cheap sex toy as well as the moral battle of whether or not attraction to men is ok."
Finally, if you want to hook up with a girl, talk to people. Nothing can prepare you better for approaching a girl than just interacting with other people. Say hi and start a conversation with the local bank clerk. When you are waiting in line compliment the person in front of you or asking them about some product they have.
Self improvement takes time and honesty. There is no quick fix. Having sex with a girl is not going to solve all your problems, it may breifly make you feel better but eventually the feelings will return. Rejection is also a part of life, you have to get used to it, it WILL happen. It will hurt, but you have us on here to vent to and hopefully you have some real life friends who can help you too.
Take some time and figure things out, stop looking for a quick fix and get rejected a few times so that when you are accepted you feel a sense of accomplishment!