It depends what stage the relationship is at.
If I get the "are you sleeping with other girls" question early on, I just tell them that I'm dating around .. But if you've been dating say 3 months and you are looking to make her exclusive then it's ok to say you're not sleeping with anyone else.
If you get the "what are we" question and you don't want her to be your gf just deflect -
Her: what are we?
You: human beings
If she keeps pushing for an answer just tell her you don't want to rush/you want to get to know her better or just be honest with her ..
Avoid talking about past relationships .. If they ask why my last relationship failed, I just say "not sure" .. I've found that high quality girls will rarely question you on past relationships. If a girl keeps pushing me for info I'll just deflect with some teasing -
"It's in the past, but now that I have your attention, we can get down to the more serious business of making fun of you"