Is it considered molestation if I don't stop it?

Something happened between me (15) and my step dad (28). It wasn't , but it made me very uncomfortable, and his was on my bare skin by my . I wanted to stop it but I didn't, and afterward I felt really dirty and gross. I told him this and he said nothing was supposed to happen and it would never happen again, and for me not to tell my mom because he doesn't want her to divorce him. This has also kinda happened a month ago but he made out with me and I told my mom but they stayed together. He told me it would never happen again that time either. Both times I didn't stop it. Is that still considered molestation? Am I bad/gross/weird for not stopping it? Should I tell my mom even though I told him I wouldn't?