Yikes! I don't know how you are going to bring it up now... I personally would have the 1st time ot happened when he was injured. In fact, I would have just brought him to dr/called dad when the injury initially happened and NEVER haf looked to begin with. 

But this NEEDS to be addressed ASAP so the kid can get into therapy ASAP before something serious happens! This is 10000% no where near behavior. 

This is a hard conversation to start but needs to b had. There are lots of people that have sexual fantasies with step moms and step sisters. You need to tell your husband ASAP.....especially if u have a daughter.

Tell your husband while u both go on a walk. I find this to be a smooth way to break hard news to ny husband about my SS. Nature just naturally relaxes you and you are away from your children and other distractions. 

In regards to your daughter, maybe you should have " The Talk" if you haven't already and maybe look into self defense classes. Not just as protection from your SS ( not saying anything will happen.....but I am a worst case scenerio thinker also) and other predators in general.

You cant keep this quiet over fear your husband wont believe you. I understand the hesitation, but as king as you approach the situation with finesse and tactfullness I am sure you both will be ok.

How old is he?!  This is vitally important.  If he's 12, it's a different conversation than if he's 16.  And what is his story? Is his mother in the picture? Why are tensions high? Has he been sexually abused?

It's not unusual for boys to be attracted to anything female, including stepmothers and stepsisters.  Apparently a whole porn industry is based on it.

Talk to his father and make a plan.  Figure out if this is relatively innocently inappropriate or if the has a history of inappropriate sexual behavior.

First, whatever you do, don't say that you did not tell him until now because you were afraid he would not believe you.  That sounds like you don't think he trusts you or that you feel irrationally guilty in some way or that he would not react like a reasonable person.  Maybe true but it will NOT start things off well - he will be defensive from jump.  Say that you hoped you misinterpreted it and were not completely sure there was a real problem until now or that you had hoped it would go away on its own if you ignored it because it was just a phase or ANYTHING other than "I have been dealing with this and could not come to you, the man who I am supposed to be able to come to about anything, because I thought you could not handle it"  Ouch......

And the other ladies are right.  SM porn is a thing.  If you have a teen boy in the house you can wear a snowsuit all day every day and, if they happen to develop that bent, it makes no difference.  They will be thinking about what is under the snowsuit.  Hey, at least he is not fantasizing about cutting off your head and putting it on the kitchen table next to the bowl of fruit.  That would be the rest of us  Nice choice, huh?  F*cking teenagers......

Talk to your husband tonight. Tell him you're sorry you didn't tell him earlier but you've never dealt with anything like this before, were in shock and did not know how to say this. Going forward keep the relationship between you and ss very sterile and professional, think teacher/student or like he's a client. If ss whips out his snizzle get your phone and call his father. Make a list of protocols that will work in your home. 

• Limit physical contact. No hugging where your breasts press against him, only side hugs.
• Avoid being alone with him when at all possible. Do not enter his room, bathroom, or be in any room alone with him.  
• Everything gets deferred to his father. 
• Enroll this kid in sports and other activities so he's too tired to run around with an erection!  

Your unrelated female presence and the fact that you have boobs is obviously a trigger for this kid. As for your dd, you're going to have to watch her like a hawk with him around. Like others have said, there is a lot of fantasy material online. If you type stepmother, stepsister, stepson, stepfather, into a search engine you'll find sexual references immediately. 

Start the conversation causally like this: “Hey SO, I think you might want to talk to SS. I think he’s getting more aware of females and his body. Do you remember back when he got hurt on the atv and I checked him out to make sure everything was okay? Well he’s gotten a little too comfortable with just pulling it out. He did it the other day and asked me about his size? While I don’t think he means anything by it, (say this part to deflect your husband from assuming you’re accusing his kid of anything) I think he needs to know that he should be talking to you about these things and not me. He could tell someone I’m checking out his privates and while we know it was because he was injured, someone else might not and call cps on me.” 

The goal here is to act like the child is innocently mistaking y’all relationship as a son would his mother, but you are worried it might be misconstrued. Leave out the part where you think his son is the creeper. That accusation will put daddy on defense mode. And this way if he does it again you can immediately tell his dad and he can jump in and correct the kid. 

These ladies are all right. This needs to be addressed, but try not to freak out (even if you are freaking out on the inside). 

Talk to him tonight as noted above. Be direct.

FIRST rule of thumb when you are with someone else child is, only the bio parents check private parts. Doesnt matter what happens...take him/her to the ER if your alone with the child... but do NOT get in that position where someone can think the worst.

 Next I never wore clothing that my dh skids could look down. AS much as my dh loves my boobs (he is a boob guy), even now I am very careful to not wear low cut stuff in my own home. We have minors still at home. It stinks to have to think that way.  I am not a prude at all nor am I a holy roller that wears clothing up to my neck and skirts to my ankles.....but as a parent well, especially with young teen boys ,,,I am very VERY careful all the times.

Walking into school...again I check what I wear before I leave the house.

I am sorry this happened to you. Its disgusting and not normal behavior in step families. Honestly I would strongly consider getting out of that relationship. Who needs that junk....