I don't post much here, but I feel compelled to post to agree with Mr Otter. I think as much as some women may be "turned off" by the idea, I know for certain women (men also, of course) don't like at all to be misled or lied-to. It will blow all trust in you if you continue to have the topic brought up, but don't tell her; also now she knows you're keeping "something" from her. Can't be good for the relationship. You can reassure her that you are not wanting to pee on her, she doesn't have to touch wet things, herself, etc.
To be complete, I'll share my story about this, I waited quite awhile to tell my current wife, but I definitely did want to tell her before getting serious -- toward thinking about marriage. We didn't talk much about sex, but once the topic of fetishes did finally come up, it was very difficult, but I told her. In my case she was very understanding about it, and together we talked about how this may have come about (not sure, myself, really). She has a degree in psych and is a social worker so it was sort of interest to her as a case study, maybe. Today, some 20 years later, she doesn't really participate; she's quite vanilla about sex. I am OK with that, and we seem to have a healthy sex relationship. In my case the DL is more about how it feels to me, and I generally try to be discreet about it when I do have desires, wear around her, or do related laundry and such -- mainly I don't want her to feel I am choosing that over being with her.