how would you respond to this?

Bumblebabe • starting new :)
So my friend (guy friend) sent me this. I've known him for about a year now except he knows I have a boyfriend and I just felt really weird when he texted me this. Why do these kinds of pictures even exists?? 
Anyway Idk what to say to him. I was just like wtf and he said he was "sorry" and "just horny" but idk :/ I don't want to hang out with him anymore but hes still my friend and he did apologize. What should I do? Should I ask my boyrfriend? I don't want him to get mad but I feel like I'm doing something behind his back. 

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