I'm not trying to get preachy, nor am I trying to lay a gulit trip on you, but you came here looking for opinions. I can only give you mine, and this is from the perspective of someone who is openly gay. Let me first stress this: I am not one of those who say, "just because you suck cocks doesn't mean that you need to think about labels." I'm sorry, but in some situations, labels are very helpful in sorting out things; this is one of those situations. Engaging in oral sex with a same-sex partner out of curiosity doesn't necessarily indicate that you are gay or bisexual. Many people tend to be adventurous, and that includes exploring the many facets of one's sexuality. Nor does engaging in gay sexual behaviour where heterosexual contact isn't an option: "any port in a storm" is a reality. Those are not what sex on the DL is. And let me stress that oral sex and mutual masturbation is sex. Actively seeking sexual partners of the same sex, or habitually accepting sexual propositions from those of the same sex, is not normal for someone who is 100% straight. I'm not saying that sex with another man is not normal for you, but I am saying that you're not 100% straight. You are willingly, even eagerly, engaging in overt sexual activity with other men, and it's not a one-time thing. That should tell you something about yourself. It may be hard for you to admit, even to yourself, but it may be tougher to keep lying to yourself: you enjoy sex with men ... you are bisexual or gay.
What to do about your girlfriend. You're cheating on her. There is no way around it. If you're supposed to be in an exclusive relationship with her, every sexual outing with one of your "DL brothas" (damn, I hate that term; it sounds like a 'gangsta' secret society of ass pirates) constitutes an act of infidelity. Are you obligated to tell her? As a partner in a committed relationship, I would have to say yes. Even moreso if you intend to continue blowing your male friends. She deserves to know what the man she is devoted to is like in his private life. Not to mention, what if you bring home a case of crabs or an STD? If you're not prepared to come clean about this, you're not ready for a committed relationship and it's not fair the lead your girlfriend on. If she's okay with the idea of sharing you, that's a different matter: her consent makes all the difference in the world. You've indicated that this situation is deeply troubling you; do you really think she can't tell that something's up? As a committed partner in a relationship, she's going to want to help you through whatever inner turmoil you're experiencing. By withholding that information, you're denying her entrance into your confidence. That's not a healthy relationship. Cool it until you've made up your mind what you really want. Sorry, dude, but having your cake and eating it too is not the way most relationships work. You have no right to drag her into a situation where you know she's likely to be hurt.
Of course, this is just my opinion: the ultimate decision is yours. But remember that there are more feelings at stake here than just yours. If you really care for your girlfriend, think about her feelings first and foremost. Either way, there will be pain. Ask yourself, is it better to get the pain over with, or should I prolong it and allow it to escalate? You already know my opinion.