First of all, please take the caps lock off!
Second, be honest with him about how you feel, and try to figure out why.
Even if you try to go along with the way he wants things, you're going to be miserable, and you're going to take it out on him and her in little ways that will make everyone uncomfortable.
Better to stop things now and prevent anyone from getting hurt more than they already will be--- which you should know that retro-active vetoes are viewed by many poly people as the nuclear option, because there is fall out.
The person who got vetoed is going to be heartbroken, and it's likely that the partner whose second relationship was ended (in this case, your husband) will be hurt by having the relationship end and resent his partner for ending it.
So, your choices boil down to
A) Pretend that you're okay with it and let your jealousy and hatred fester
B) Demand he ends things and deal with the radioactive fall out in your marriage, and knowing that you broke the heart of a woman whose only crime was loving your husband with your permission.
C) Be honest, ask for some time, and use that time to work through your jealousy and figure out if your issue is with him having another relationship or if it's his choice of partner.