After reading your So What Happened, my original thoughts changed.
First, since this little girl is the daughter of your boyfriend, her mother's actions are not your business. What happens in your home is your business, but you really can't confront this little girl's mother. You can parent your own son, and establish some basic rules in your house. But this child is not your step-daughter. She's your boyfriend's daughter.
You can also hope that your boyfriend parents his daughter effectively. How he responds to this situation will tell you a lot about him as a parent and potential future spouse. The two of you should not have approached his ex. That's on him.
If you think you're going to stay together, you have a long long time before the kids are grown and out of the house. They're only seven now. If your boyfriend teaches his daughter right from wrong, privacy, respect, saying "no", and morality, and if he is willing to confront his ex and make sure that his young daughter is not kissing boys at her age (in any manner), well, that says a lot. It also says a lot if he needs you to come with him to confront his ex, and if he allows a parenting discussion and fight to take place in front of his daughter, and if he doesn't step up right now and help his daughter establish some boundaries and some personal self-respect. His daughter is going down a rocky path. There are several people in this scenario that need counseling.
Your loyalty is to your son. If a boyfriend's daughter is teaching him this stuff, and sharing the kind of information that she did (telling him she's done this with other boys at school), how are you teaching him and helping him? I know we can't put our kids in bubble-wrap, but a child who is willing to teach a parent's new partner's child about French kissing may not stop at that. You should be vigilant where your son is concerned. The girl may be sweet but I think she's crossed a line.