Open Relationship?

Recently my boyfriend has been expressing his openness to an open relationship. I personally don’t crave one and know an open relationship isn’t for me. Every time we discuss it he explains that he’s still attracted to more masculine men since I’m more on the feminine side but he refuses to follow through with his feelings because he knows he’ll lose me in the process. It makes me feel guilty because I want him to be happy and if I’m not fulfilling his desires then he shouldn’t have to burden himself with me. I don’t know what to do.