Swinging, open relationships?

I am engaged to a wonderful man, but there is a BUT lol. He was in an open relationship as he calls it with his ex wife and also with his first wife. He considers an open relationship to be with couples an also with other single people alone. I have never been into anything like this. I am very scared and tend to be a jealous person i could use some help here how do you or “I” begin to handle this? Also what tends to be the reasons for some one to want other sexual partners. I really have no desire for that but he does. Even though he says i fulfill him which i feel isn’t true if he wants others..Some input would be very nice