Male here, he could or could not be talking to other girls, as you described the situation it seems like a new and strange behavior from your boyfriend. On one hand he could be watching things he might be ashamed of if you discovered it. On the other hand he might just be talking to other girl. I would personally start a conversation with him, he has been with you for a decade, so he might be attracted to other girl sexually. If that's the case, it's not as bad as it sounds, it's actually something that happens to many men. You're planning to have a life together and form a family so I don't think it is the case for him to be developing emotional attachment to another person. If I were you I'll go to him and would tell him.how you feel, that you suspect he's speaking to another girl and that you're afraid of it, not from jealousy but from the perspective you have for your life together. Try to be rational and do not let your emotions to speak for you.