What am i suppost to do, i love him but he wants me to share. please tell me why. am i not good enough?

I love my boyfriend but today i felt so completely upset. He had only just come back from rehab (because he has alot of issues from when he was in the marines) and i was expecting him to kiss me and hug me and tell me he missed me, but instead he told me he met a girl in rehab. He told me that he would like me to share him with her for a bit so he can help her get better. I told him he could help her but i wont be letting him date her. He then said so where breaking up then and i was like if thats what you want. . . he then called me a lier because i told him id fight for him if we ever broke up and i wasnt. I told him that there wasnt anything to fight about if this is what would make him happy. Thats when he said he was just joking about breaking up with me, he just wanted to get me to react. He's been to this girls house and met her mum. He said that it makes him sad asking me to share him. He said that im the love of his life and that if i let him do this that he would marry me.