My son's friend came on to me, and i got turned on.

My son is 16. His best friend (15) is openly gay, and tbh it's not even a subject that comes up : It's 2014, people can live however they want.

One day, his best friend, who i'll just call Zack for the sake of this discssion, was at our home since they had a school project or something. My son said that he forgot the project file at another friends house, who happened to live about 25 minutes. My son said he'd be back, and Zack was sitting on the couch watching TV and eating chips.

I came downstairs and sat on the couch too, cause i took sick leave that day and had nothing to do. his mom was out with her friends. Anyway. So i was watching with disinterest whatever Zack was watching (it was stupid teen drama show). Out of nowhere, he dropped a few chips on my crotch. He reached over to pick up the chips, and started rubbing up on my...well you know. I said "Zack, what the hell are you doing?" to which he just snuck  his hands down my shorst and grabbed it, and started jacking it. I did get slightly aroused but at the same time i knew that this WILDLY inappropriate, and immediately got up quite alrmed. Before i manged to get away, he almost managed to pull my shorts off. Puzzed, dazed, and just confused, i went into the other room, to hear him yelling stuff like "Please, i know you'd love a little a head" and "i just wanna try it with an adult". I stayed in the other room till my son got back, and things went back to normal.

I'm ashamed of getting semi erect, i really am. I've been feeling horrible, and have spoken to this about no one. I'm confident about the fact that had i not gotten up, he would have tried oral sex on me. Mayvbe it was the abruptness of it? Obviously i wasn't attracted or anything but i have no idea why i got a semi boner.

What do i do? Should i tell his parents? Should i tell my son?