Like other people have said, it's better to be honest and be up front. If you aren't, it'll raise a lot of questions later. Now I'm not saying, first date to tell her, but once you've been together for a while, it's time to tell her. The best way would be when you're comfortable, to ask if she has anything that excites her and relate it to that. Or talk about sex and mention that you're into this. Tell her that it's been a part of your life for awhile. If she's curious, offer to show her some videos, but I wouldn't start with Ultimate Surrender or something. Start small, show her the athleticism. Like DWW or IFW. Tell her what you like about it. Then progress and explain. Be open and honest.
If you aren't, when you get serious, she'll wonder where the money is going and she'll know you're hiding something and may think it's an affair or something. She may surprise you. And if she doesn't like it, if she's the one, she'll still love you. Who knows, you might even stop watching it for her. But honesty is ALWAYS the best policy for this stuff.