This is essentially the difference between being a gentleman and being “that asshole.” Womanizing assumes some sort of deception. In contrast, dating multiple women is about being indecisive and actively avoiding monogamy. It has zero to do with duplicity or running game. 

She doesn’t need to know about how your still-beating heart was torn from your chest, or that you prefer to sow your wild oats with a minimum of five women simultaneously. What she does need to know is that she’s not the only one. While difficult to say, telling her you’re seeing other people conveys you’re not looking for a commitment, and leaves the decision to get involved (despite the fact that it’s unlikely to lead to a relationship) up to her.

While it doesn't rid you of guilt if you hurt her, it does give her the option to opt out if the terms are not agreeable to her. Present her with the facts, and let her make the decision herself.