Is it still going on? I thought I saw you may not be having contact with him/it may not be happening anymore. If it is, you can call your state's (if you're in the US) Child Protective Service (or your state's equivalent) toll free number and make an anonymous report. Some states even allow you to do them online. The key part of your situation is that you were uncomfortable and that you told him no, plus the fact that you are not legally allowed to consent. Child Protective Services may not have enough to validate the guy for sexual abuse on their own, but a police report can be generated and law enforcement deals with different burdens of proof so they have more options on how to pursue charges on the guy than CPS. You mentioned his girlfriends leaving the kids alone with the guy. Do you have valid concern that there is sexual abuse (or other abuse/neglect)? If you are truly worried about these children, you can call in a report (can be anonymous) on him with concern for those kids. Even if the report is just something like: "I am underage and he has pinched my nipples multiple times above my clothes, despite my asking him to stop and telling him I was uncomfortable. This happened more than once, in front of other people, and while we were alone. Other children are frequently in his care with no other adult supervision. These children are not biologically related to him (girlfriend's kids)."