I completely agree. 

OP, for a year you’ve been trying to get him to tell you what was going on. He had every opportunity to be honest and forthright. Now he’s smirking and saying he wants an open relationship. No. He wants to cheat and keep you miserable. If he wanted to open the relationship, then he should have discussed it with you before he started fucking around. 

You don’t deserve this. Take your love and give it to a man who loves you back.