Maybe he's just curious - particularly if he's never been around breastfeeding mothers. I know my FIL and MIL were weirded out about it - but screw them, I BF where and when I wanted to. 

On the other hand, breasts ARE sexual, too. I'm sure he's attracted to women's breasts - why wouldn't he be (if he's straight and that's his thing). Explaining that they're not sexual isn't quite right either and is likely to be confusing to him. It's all about context. Body parts can be multipurpose! Sometimes my hands are sexual and sometimes they're used for typing! And sometimes they're used for typing sexual things! Male sexual organs are used for urinating as well as for sex. 

What makes you think he is sexualizing it?

Just ask him straight up why he’s searching breastfeeding on YouTube. Ask him if there’s something he would like to know about breastfeeding, because maybe you can answer it for him. And if not, you’ll do your best to get a medically backed answer. 

On the flip side, and all my years of experience breastfeeding back to back babies, there’s that small percentage of males that cannot get past breasts as a source of food and not sexual playthings. 

15 yr old boys find everything a sexual turn on.  My advice was unless he's behaving inappropriately, leave him alone, he'll get over the rather prurient and odd obsession with breastfeeding.  Breasts, as another poster pointed out - are dual function, they are for pleasure as well as breastfeeding. 

can get turned on in math, so yes, breasts are a turn on. And breasts are sexual, there is no denying that. As a mom I would be irritated that you gave him your interpretation on what breasts are for. They are for pleasure and for feeding infants. Misinformation can cause shame and make a teenager feel like their natural sexual feelings are wrong or perverted. 

As much as some in our society want to desexualize the breast ... the fact is that in our society breasts have been taboo for so long that they have become hyper sexualized.  Sure, the collective American we understands what the breast is for. But it is also a gender specific characteristic with reproductive and sexual appeal.  Which causes quite a conundrum for many American males.

Having been a long time expat and a product of the Americanization of the breast it took me quite a while to tune myself for the less taboo perspective on boobs.  Even though I have spent more than 20 years of my life internationally and been to the french Riviera, etc, etc, etc....  I still have to consciously subdue my reaction to bare breasts in public. 

The most shocking example I have of this was in Qatar.  We had driven across the dunes to the inland sea and parked on top of a dune overlooking the beach.  A 4x4 pulled in right next to us and two European women climbed out, stripped down to butt naked and proceeded to sun bathe.  Apparently they thought being next to us made it okay since we were in a Middle Eastern country.  Several Arab families arrived and set up near us and the two ladies never covered up.  I was aghast that they were so clueless and brash.  I had to approach them and tell them to get dressed and to respect the culture of the host country.

That and I could not quite overcome my American obsession with reacting to other cultures more evolved perspectives on the naked human body in public settings.  I had to very consciously not look.  Hey, I appreciate beauty, what can I say?

It doesn't sound to me that the 15yo is being particularly inappropriate in his research.  I would do the same thing at his age.  In fact I did.  The encyclopedia, National Geographic Magazine and the Sears catalog were the research material on breasts back in the day.

1. What sparked the conversation about breasts with your stepson?

2. Who is catching him looking at breastfeeding videos?

3.Are the videos sexualized or just moms feeding their kids?