I think sometimes it's easy to think that if you love someone, not telling them something about yourself is lying by omission. But it's okay to have inside thoughts. You don't have to volunteer every single thing about your inner life. And doing so, especially unsolicited about a strange porn preference, can really signal that you're not a person with good judgement.

Is this... interest... a major part of your life? Are you spending hours a day spanking it to this stuff? Is it affecting other areas of your life? If not, just keep it up yourself.

Maybe one day she'll ask what your fantasies or porn preferences are in an attempt to spice things up or cater to your sexual interests. At that point you can decide. And if you want to tell her, you can preface it by saying you're a bit embarrassed about it so she doesn't laugh.