Originally Posted by SouthernGal
In answer to the original question, you can't make him see the danger posed by others. You can however talk to him about the warning signs, set up boundaries for specific behaviors (no asking that he leaves you, for instance), and request that he not share with you spoken opinions about you. It would be soooo nice if he chose to defend you, but that might not be his personality, or he may feel she is only venting. Regardless, there is no need for you to be hurt by it. I am very sorry you've had to deal with this, and I really feel for you.
Well said, SouthernGal. I would like expand just a bit on what you said about setting boundaries. Boundaries get mixed up with rules quite often and I think it's important to clarify the difference between the two.
A rule is set to govern behavior, most often including someone elses behavior. "Use condoms with all lovers besides me" or "I'm the last person you kiss before you go to sleep" are examples of rules governing other peoples behavior. "Tell me you think what she said is unacceptable and 'realize' how dangerous what she said is" would be another. Rules are made to tell everyone involved how to behave.
A boundary is essentially letting people know there is a limit to what they will endure. "Don't tell me when your girlfriends talk smack about me and you are ok with it" is a boundary. It's not telling someone what they need to do with their life, it's telling them not to tell YOU about it once they've done it. A boundary is personal and only regulates what someone can do with YOU. "Tell me if you have unprotected sex with one of your partners so I can make an informed decision" is another good example of a boundary. Instead of telling someone else what to do with their genitals, you are simply insisting that YOU not be dragged into their bareback decision without your knowledge.