Hi,
Your husband does not respect you. This is not a good sign. Don't do anything that you're not comfortable with. If you refuse, he probably will get another woman on the side because he can't get enough sex.
My friend OronD suggested for you to agree for a threesome provided the 3rd one is a man. What happens if he agreed? Are you going to do it? Your man is kinky, I don't think he'd mind if the 3rd person is a man. So better not make any suggestions like this.
There is definitely something wrong with your marriage. You both need marriage counseling. If after the counseling he still does that kind of stuff again, then I would just say goodbye to him and let him have all the 3-somes, 4-somes,10-somes he wants. This is not a marriage that will last a lifetime.