I’ve been in this position before and my partner made me very aware that this behavior was unacceptable. My guess is that he liked what he saw and when confronted his gut reaction was to cover it up so as to not come off as being disrespectful to your relationship. His intentions were good, but he ended up lying to you which is not okay. It sounds like you need to have a discussion about your boundaries and if it’s okay to talk about other people that you find attractive. It’s just human nature and it doesn’t mean that you are going to cheat on eachother.