My boyfriend of 4 yrs think its ok to have with another woman and I dread it. ?

Hey ppl..My boyfriend of fours years keeps talking about 3somes but when i mentioned to him i would like a foursome with another guy and a girl, he blatantly said no! Moving along from what I just said, the main issue is that this morning he actually told me that he would have with another woman cause its not an "emotional" thing for men. I was deeply hurt and told him so. I also retaliated and told him that if he did that , then i would sleep with another guy and he replied saying "i know u wouldnt do that cause u r not that kinda girl".. i was like wtf! im so angry and sad right now cause iv been with him 4 yrs and now, he seems to be really serious about this " sleeping with another girl thing" He also told me that he wouldnt tell me if he slept with another girl cause he didnt want to hurt me cause he really loves me (we have plans of getting married, to which im having second thoughts now).. I dont understand.. Why would he want to do it in the first place if he didnt want to hurt me?