<human>: Provide a brief overview of the text.

We've been together for 7 years and are planning to get married in the next year or two. Our relationship is great other than this issue. We have fun together, rarely argue, have built a life together and have a bright future ahead. We were each other's first for everything. A problem in our relationship had been that he feels he is missing out by not getting to have sex with other women. He has been having doubts about getting married because of this. I have been trying to be understanding and open about it. We had a threesome a while back and we both loved it. It's something that I'd love to do in the future. After the threesome he told me that the urge to go out and explore was gone, but then he said that it's crept back up on him lately. 

I just don't think I could let him go through with it without being hurt. It's got to do with a lot of my own insecurities. I've lost a lot of weight and I'm still losing weight, so that's a big issue. He has also said that he doesn't really find me 'sexy' even though he loves me, enjoys sex, couldn't imagine marrying anyone else, etc. He basically says that he's pretty sure I'm the one that he wants to spend his life with and I don't have to worry about him leaving me for anyone else, but he doesn't think that he would be happy if he couldn't have se with other women. I am trying to believe him, and even though he's never cheated and never given me a reason not to trust him, I'm still scared he'll like someone else better than me. 

I don't know how to tell him that I'm not okay with him having sex with anyone else without me. I don't want to make him feel trapped (which he says he does sometimes) and make him ultimately leave me. I don't want to be the crazy, jealous girlfriend. I just want to be enough for him. I don't know how to make him be okay with not having an open relationship.
<bot>: Fiancé wants an open relationship despite me being okay with threesomes, I'm uncomfortable with the thought and don't know how to communicate it to him without sounding crazy, jealous, and possessive.