My hubby-to-be flirts with every woman

This week’s topic

I have been dating this guy for two years and we now plan to formalise our relationship. However, I am worried about his flirtatious nature. He flirts with everyone including my girlfriends. He even tried to hit on my sister once and although she did not give in to his demands, I could not help but wonder what would have happened if she did. This habit annoys me but I avoid raising it with him because I may come across as insecure. I love him but I am not sure I can trust him fully. Please help.