Dude, you've got to cut yourself a break. You are 21...just a kid relatively speaking. I know you said you are in love and want to marry your girlfriend, but believe me, at your age, that could change practically overnight for a myriad of reasons. I think you really ought to put those plans on hold for a while, since you mentioned that when you have open and honest conversation with her about your true feelings, you often break up. That's not a good sign for a mature, marriage ready relationship.
Then, add in the component of "organized religion", and it's no wonder you're making yourself nuts. Organized religions thrive on their ability to inflict fear and guilt into their congregations. Once you become sexually active, it's a pretty daunting, if not futile, task to revert to being celebate again...kind of like shutting the barn door once the horse has already gotten out. Being curious about ****ography is also perfectly normal, and as long as it doesn't interfere in your regular activities or cause you to alter your life or relationships, there's no reason to freak out about indulging in the occasional viewing. Just remember that what you see is SIMULATED SEX, the people are ACTORS, and they are getting PAID MONEY to make things look unbelievably fabulous.
Now, if you look at how you phrased your explanation, you talked about "getting into religion", which has absolutely nothing to do with exploring your faith or a belief in GOD. This just screams of human men, not men of God, thumping their bibles and preaching fire and brimstone for all the unclean sinners, including those who engage in pre-marital sex. Again, control via fear and guilt. The worse these men can make you feel about yourself, the more important and powerful they become. There is nothing worse than a man that boosts himself up by falsely placing himself on a pedestal in the name of GOD.
At your age, you have so much life ahead of you...you are still finding out who you are as a person, and it's a tough endeavor, especially with all those naturally raging hormones. The thing you have to keep in mind is that you are not experiencing anything abnormal, but you really need to cut yourself some slack. GOD is not vindictive, and there is no shame in experiencing sexual feelings that are part of the mind and body that he created for man in his own image. If you're not being sexually active because you are emotionally not ready, that's one thing, but I think you need to re-evaluate just why you are reverting to this reclaimed "virginity". You really need to ask yourself why are you putting yourself through this? Is it what your girlfriend wants? Is it parental expectations? Frankly, I don't think you would be nearly as conflicted as you are if it was truly the right path for you to be following.
I'm not saying that you need to be running amok, sowing your wild oats at every opportunity, but maybe you could pursue some SECULAR counseling, that could give you a little guidance...free of the fear and guilt factor.
Best of luck to you, and please let us know how you are doing.
•On Disability for: Chronic Migraines, serious Back and Knee problems (will need surgery eventually), moderate Depression, Anxiety/Panic disorder, TMJ, stomach problems
•Divorced, 45, spawn-free
"THE WEATHER IS HERE, I WISH YOU WERE BEAUTIFUL." -Jimmy Buffett