Another hocd question, please answer

Hi,
I know that people would have read these sorts of posts maybe a thousand times and in the end they all become similar stories but for some reason my brain won't accept any answers I read of other people's threads untill I fully explain my situation and I know the answers that I'm getting are fully relevant to me. So I'm sorry if you may have also posted on other peoples threads but please answer as it will help me loads :D
Ever since I was a little kid I knew I was straight, never questioned it because I didn't need to. Had girlfriends like a little stud muffin since I was in junior school. Even when I went to an all boys school I had girlfriends, and this is the story throughout my adolescence and education. Although I've had a few relationships they have never became deeply intimate, I.e I haven't had sex yet. This, I knew at the time, was probably down to the Christian upbringing that I had growing up. The thought though of having sex with any of my girlfriends at the time was very appealing, if it hadn't been for my fear of god.
Anyway, so since leaving uni and getting a full time job, I'm with the most amazing girl. Ive been with her for a few months now. She is funny, smart, my stomach errupts with butterflies before seeing her, I could have not eaten for the whole day and not feel hungry at all when I'm with her because my mind is totally blown by this girl, in short, I think I love her. But there is a problem. For the last couple of months now my brain has been telling me that I'm atracted to guys, it's infuriating. I have like pop-ups in my head that I can't seem to close where mentally I picture an all male sexual scene and certain areas start to get excited. I get scared that I might be gay and so I watch heterosexual/lesbian porn which gives the same but greater reaction as mentioned above to prove to myself that I'm not gay at all. These pop ups only ocourr every so often and when I feel like I'm straight again, it makes me so happy. So happy. I get errections from just thinking about sexual encounters with females and this makes me feel extatic. In my sleep I dream about me and my girlfriend so I counter act my brains constant attempts to throw these images at me with "well my subconscious is straight, I want to be straight and really I know I am" but I can't seem to win. When I see a guy, I don't feel any attraction, at all. I don't find myself longing to look at men, intact when I feel like I'm my "straight" self the idea of a gay relationship kinda makes me a little homophobic. But my brain throws these pictures that I battle against so hard in my head, if I try to ignore it I feel depressed, ive wanted to hit myself to give me other things to think about because that's when it seems to affect me most - when I have nothing occouring in my thought processes. At times like this, I struggle to think of girls and the images that usually make me feel straight and really happy. For example, when I finnished playing rugby the other day, I was extatic with the way I played, my mind was on the game, didn't attract me at all that I was in a room filled with naked dudes. I actually got a text from my girlfriend at this time and I couldn't wait to see her. But sometimes, in these horrible filled mini windows, my head seems to endeavour to think that's what I want?!?!? But I don't, but then my head will make me feel like I could never satisfy a woman completely because I'm not meant to be with one - but I want to so much! This has also affected slightly the relationship I have with my girlfriend. We were together one night a while back after going out for a couple of drinks, we came back, turned the lights off but nothing happened down there. Even though it was moments like these I've dreamed of, and continue to dream off! Now I feel scared whenever we have a night alone together, mini-me won't rise to the occasion. This may be down to a real fear that, as a virgin, I may disappoint her expectations, because I want to show her how much she means to me. Also another fear I still have of god. I think the former may well be where the hocd, if that's what I have, spurred from. Guys, I don't know what to do, just writing this makes it feel better, im not great at communicating my feeling about anything verbally, but I need reassurence that I'm not gay like my brain likes to tell me. I hate this feeling, I hate feeling depressed, and i want to be with my girlfriend and stay in a mindset without these flashes of homosexuality, because it's not me! Im sorry I've written an essay, my head wouldn't feel right if I didn't write everything down. If it helps, I have always feared that I have OCD, when I was younger, I couldn't stop washing my hands thinking they where infected. Similar things happen now but to a lesser degree. Help, if you can, much appreciated. Peace out.