What should I do

So my fiancé is very very dominant and he ‘doesn’t play well with others’ when it comes to me, he is a navy man and he’s that type of guy that watches me across the room, will walk up and place his hands on my hips and muzzle my neck, and has me sit on his lap whenever there are guys around who he thinks are checking me out. My issue is he is away a lot and I am a very sexual person. Sex is unbelievable when he is here but he’s gone sometimes up to 6months and I need to have sex 3-4 times a week at least. I brought this up with him because Ik when they dock he probably gets something from girls he meets with his buddies (I come from a navy family I’m not naive) and I have always been frustrated by that but it’s what happens.

My issue is one of his navy buddies is retired and he has been making advances, I have had a threesome with him and my fiancé once and it was incredible. I don’t know if I should sleep with him (JUST ONCE) to sustain myself because I’m fiancé has been gone for close to 8 months and I’m dying. Opinions please!!!