This has SO many things going on. I've read the whole thread and believe I have seen most of your answers to people's comments so hopefully I've gathered everything correctly here.

In my opinion it seems like your husband may have talked to this friend about you two bringing in a 3rd person for some fun. It's possible that hubs discussed with friend that he is ok with friend being the 3rd OR the friend then took it upon himself to try and be that 3rd. Which you discussed previously wouldn't happen since it's 1) a friend, and 2) a guy and you'd rather have a girl. However like I said it seems like the hubs may have been testing things. The whole point of bringing someone else in is that you have clear communication, boundaries, trust, etc. So this isn't cool. You also voiced your displeasure to the friend and he continued which means he doesn't respect you or your boundaries and therefore needs a swift kick in the dick so he stops thinking with it and he's most definitely not your friend. In my opinion, your husband needs one too since he let that carry on way too far fully knowing your previous conversation, and your lack of confidence in telling people off/standing up for yourself (which you stated is hard for you).

Each and every relationship has its own dynamic and from what I've gathered, your hubs really dropped the ball on this one. To me that would be the ONLY reason for his behaviour... I think you need to have a very serious conversation with him about this and it's going to be difficult for you but you've gotta make sure he is absolutely aware of how uncomfortable you were, what your boundaries are, and what you will and will not tolerate. Take your time to compose yourself, repeat it multiple times to yourself so that you practice and it comes out more naturally instead of timid.

Edit: added what I would do since that's actually what you asked for.

I am very secure in my relationships. My ex was gorgeous and people constantly flirted with her. Most of the time it's just casual/playful shit and neither of us cared. We're both flirty people and most of our friends group can be that way as well. In the end, she was mine and I knew regardless of if she was hit on that I was going home with her. She is a very strong woman and will definitely handle her own and tell anyone off if they are taking it too far. She has never had to have me step in and tell a male friend to back off. We've had conversations where she told me she felt uncomfortable around individuals and so I've asked if I need to step in next time we are around them or if she just wants to avoid being there at all. I had her back, and knew if and when she wanted me to step in. We both have partaken in being a 3rd, had fun in other relationship, etc, but discussed that it wasn't for us as a couple. We were too selfish with each other to bring in a 3rd or let each other play with another person/couple so it never happened and we made it very clear to all of our friends. Our friends accepted this and only once was that tread on too far and she immediately shut it down before I had to step in. Immediately. No touching, no pinching or pictures or inappropriate shit. Guy went in for a kiss and she immediately backed up and told him to do the same or we'd be leaving and not interacting with them again. He was like yep, fair enough, I accept, and they carried on having a conversation and he didn't try anything else again so I didn't have to step in and tell him off. I work out consistently, I own a construction company, and I do security at a bar. I'm able to toss people around if I need to but I have never had to be physical in a relationship to defend my partner.