I'm more concerned about who the 7 by 7 in cock belongs too. You are 17. In some states a guy 19 or a male 59 are both considered adults and you a minor and in a court of law, there would be no difference. If you are really straight, why torture yourself? But if you are gonig to self massage your butt, you need a dildo or something similar. A tooth brush is dangerous, much more dangerous than a penis of any size. It can actually puncture something. And for God's sake don't try anything that is glass. And you need the right kind of lube and lots of it. I am hoping that this guy you are consdering butt sex with is someone on your level that is just as concerned as doing it right and not hurting you as he is getting personal pleasure. There are some threads here that tell exactly how to do what you want. I don't remember which ones. Maybe someone else can tell you how to find them. Please be careful with yourself. It would be embarassing to go to the doctor or emergency room and try to explain how that toothbrush caused this medical emergency or how something got in there and you can't get it out.