im not even gonna answer this but you should dismiss it
I think the fact that the teenager in question is allowing himself to be "caught" in the act by you - especially since he had a picture of you on the stairs - should be telling you that he is looking at you as more of a "pin-up" poster than as a potential sex partner. He may "know" that you are his dad's girlfriend but he sees you as "fair game" since you and his dad are not married. (And if you think that he's not aware of any sexual activity between you and his dad, think again!) Calmly tell him that he is too young for you and that there really could never be any type of relationship between the two of you. Make him aware that, if he continues to masturbate in places where he KNOWS you'll be frequenting in the house, you will speak to his father about sending him back to his mother's house. Ask him to politely confine his masturbation to his room in the future and not to use any pictures of you as his "fantasy material". (You probably should do all this in the presence of your boyfriend so there's no misunderstanding between the two of you later on.) On a personal note, you need to get over your embarrassment about a teenager's normal behavior - i.e., his masturbation - you don't think his dad didn't do the same at his son's age?
How about you not feeling and behaving like a victim pf sexual assault. You have obviously raised teenage boys! Get a grip! All 14 year old boys are horny all the time and they go through a stage where they lust for their mothers. Nothing comes of it unless THE PARENTS overreact and make it a huge ordeal. He is a boy for heaven's sake!
Yes, the pictures of YOU, might be Strange/odd but the Masturbation certainly isn't. In actual fact if he WASN'T masturbating, there would be something wrong. I think that you might have become his favourite 'Pinup', all boys- of his age- have them. Get his Dad to speak to him, 'Man to Man, and explain these 'things'- I'm sure that's all it needs. All Young Adults need guidance, not a Telling Off, nothing more. Let us know how this, turns out, will you.
14 and down and up depending on the child's body chem and possible high testosterone
like to beat the meat allot i did all the bloody time
now to your pic that needs firmly addressed
it could if it hasnt already to obsession
spying at private times or showers lots of bad behaviors
possibly depending on child trying to make it come true
his father needs to and you have to force him to address this now
and it may be to far and need intervention of a counselor
he needs someone to let him know spankin his monkey is normal and fine but obsession is bad
good luck hope it gets better