While on the 13th MEU, I was sexually assaulted by a Sgt whom I had thought was a decent person. I thought we were just friendly. We were on ship together, in a workspace with a couple other Marines in there. He asked me to go into the hallway to talk to him. While it was out of the ordinary, I didn’t think anything of it. He had never done or even said anything that would make me think he would assault me. We stepped into the hallway and he started to say “Sex is a need and cheating isn’t cheating if my wife is ok with it.” I started to say “Ok, this is weird -“ When he cut me off by forcefully pushing against the wall and grabbing my breast. I pushed him away from me and he briskly walked away. I made my way back into the workspace feeling confused, disoriented and upset. A marine in the room asked if I was ok, but it brushed them off by trying to forget about the incident. I wish I could really forget about it.