DanielC, I am married to a witness. The witness religion affects every aspect of their life. Even if she isn't a strong witness now that switch can be flipped at any time. Usually as soon as you have relationship problems, married or not, they get really into the witness stuff. Life will not be the smooth ride you hope for or deserve. This isn't even mentioning the family, the congregation and the elders that will have their nose in every aspect of your relationship. Yes, every aspect, including sex and what types of sex are acceptable and clean.
Remember, her status now is probably weak because she's dating you, a worldly guy, but that weak status could change at any point - usually happens with any stress that comes into their life or if you progress to have kids. You can almost gurantee that if you ever have kids she will 100% do everything in her power to make them witnesses. Trust me, that's a mess you don't want to deal with.
Good luck Daniel.