i don't think you have a reason to be embarrassed.. it's only natural, i mean being nude. and after all, he's your boyfriend. it's just that we are raised to believe that nudity is wrong.. i would be a little embarrassed if it was a complete stranger, though..
Last time I checked seeing your gf naked was part of the deal... Where would the fun be otherwise. 15 is a little young, but you guys should be comfortable seeing each other naked atleast.
Firstly, I can understand why you felt embarassed, but it would seem that your boyfriend, although maybe a little bit suprised, was quite cool about the whole thing. Trust me, he'd of been very pleased to see the whole thing, you don't need to feel like it will in some way put him off you. To be honest, he's more then likely going to see you as being a very "let your hair" down sort of girl. And in regards to sexual stuff, if you didn't feel like doing it at the time, then it's a good thing that you shouldn't.... regardless to the thoughts going on in his head at the time (I can virtually assure you that there were A LOT of thoughts going on in his head haha). But, it'll be good man. Just chat to him =)
tell him he can't come over until he learns to ring the doorbell. - other than that, it actually sounds kinda cool that you were able to just chill. If HE BRINGS it up, put your shoulders back, your chin up, and smile. Its his fault, not yours. he embarrassed you. but don't act like you are embarrassed. act like you are proud to look the way you do. God created you, girl!!!
You're 15, yeah it'd be really embarrassing for your boyfriend to see you naked but I'm sure he thought it was hot and in a weird way it kind of showed you who he really is. If he was only with you because he wanted to shag you he would have tried something and not just stared dumbfounded for 25 minutes. So you know that now at least, but like everyone else said just talk to him about it because he may tell his friends he got you naked. -Good luck
Since he didn't break up with you, he probably enjoyed the view. If you really want to talk to him about it, try and be relaxed about it. The more relaxed you seem, the more relaxed he is.