to be offended or not.....

My boyfriend and I have been together for over 3 years. I’m younger than he is and have a way higher sex drive. I’m usually the one always up to have sex to say the least. Today however, I went into the bathroom because I had to use it and the shower was on so I assumed he was already in there. When I walked in, he quickly closed the door on me and then hopped in the shower and said I can come in. I asked him why he closed the door on me since we always randomly walk in on each other. He told me that he was masturbating and had gotten embarrassed that I caught him. I immediately felt like shit and asked him why couldn’t he just tell me he wanted to have sex. I’m always down to have sex so I was hurt that he resorted to masturbating instead of having sex with me. Am I wrong for being offended? (Also, I’m not against masturbating by any means. Its just that if I’m there then why not have sex with me).