How to convince my boyfriend to have an open relationship?

Hi. I have been in a relationship for almost 15 years now were both in our 30’s the sex has died in our relationship and we been distant lately he goes to work and falls asleep on the couch. i have fantasize about being with other men i have brought the idea of having an open relationship but he refused. i think it... show more Hi. I have been in a relationship for almost 15 years now were both in our 30’s the sex has died in our relationship and we been distant lately he goes to work and falls asleep on the couch. i have fantasize about being with other men i have brought the idea of having an open relationship but he refused. i think it would spice up our relationship sleeping with multiple people i even thought about experimenting with other women which i never have done before and having three and foursomes. I would never cheat on him and he would never cheat on me we love each other to not throw away 15 years. How can I convince him to change his mind though?

Thanks
Update: We been through alot to end 15 years maybe we will look into couple’s counseling.
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