First of all, no one's going to bat an eyelash that you have a female cofounder, and if an investor decides not to fund you because you have a female cofounder, then it is not an investor worth taking money from.

Second of all, if you don't list her as a co-founder now and you have interest from investors, what are you going to do, lie about it, or say "oh, shit, I am sorry, we forgot to list one of our cofounders earlier?" Regardless of how you think having a female cofounder makes you look, forgetting to list someone as a cofounder or lying is an immediate disqualifier.