When I'm with my girlfriend and things are getting hot and heavy, she always says "Don't do something you wouldn't want somebody else doing to your sisters." My sisters are 7,9,13 years old if that matters.

That left me in a daze and I didn't know what to say, so I just said that I'm not going to be a father yet and avoided the question. What should I say when that comes up?

Edit: We're both 17 and have been dating for 2 months. We haven't had sex. I know her brother (played football with him for 2 years) but he doesn't know we're going out