Boyfriend getting married to another for a green card?

I know that alot of people do this, but for me, idk its just a foreign concept. Me and my bf have been together for about 6-7 months. We're both African,I was born in the US, therefore a citizen, he was not. About 1 month ago, he asked me to marry him for the sole purpose of him getting a green card. I told him no. Idk to me that seems like so much to ask from someone. I thought he would drop the topic all together. Well last night he tells me that his cousin has found someone from him to marry. We are in GA, the woman is in TX. Of course I got very upset and have been ever since. I know that when your back is against a wall, sometimes people do things that they normally wouldn't do. He has been here for about 7 years, he says he's filed all the papers, spoken to immigration lawyers, and nothing.....Im torn, I dont know what to do. Right now I feel like if he goes through with it, then I will break up with him, but Im not even 100% on that. Idk I guess my question is, am I over-reacting, am I right to feel this way? Should I be maybe more understanding or compassionate to his situation? After all I was born here so I have no idea what the immigration process is like, but I've heard alot of times it is unfair. Any advice would be helpful, thanks again