I understand about the jealousy thing - until recently I have never considered myself the kind of person who could cope with anything non-exclusive, because of my rampant jealousy.
I do however have an FWB who I see every few weeks, and bizarrely it seems to work. (I did have two for a while, but recently binned one off as his behaviour became slightly dodgy - he was hospitalised and I didn't hear from him for days, and started to vaguely suspect he is actually seeing someone who doesn't know of my existence - and no way would I countenance that on any level!).
But the other one - he is actually an ex I was with for quite a while (me and my exes!). Though the relationship was in many ways shit, the sex was always absolutely stellar - definitely the best sex I have ever had - and I have had a lot of sex, one way or another!
We generally message semi-regularly anyway (just friendship stuff - how is your day, updates about mutual friends etc) and when I start to miss sex I invite him to come and stay, usually for a couple of nights. And we just hang out, and I cook for him and do his washing and he does any minor DIY I need, and most importantly we have sex, and cuddles and all that..... We both know we aren't exclusive though we don't talk about that in any detail......
I will obviously call a halt as soon as I meet anyone I am remotely keen on pursuing things with. And he would too - I do trust him to be honest about that - though I also believe he is only dating on an explicitly casual basis at the moment so it's more likely to be me getting into a relationship that him, if you see what I mean (though I know you can never say never, and I am totally cool with the possibility of him falling in love with someone and ending things with me - I would be genuinely happy for him if that happened, I think!)
I never expected to feel like this about him - it's strange how things change over time..... I do love him but I am definitely not in love with him, if you see what I mean.
But how we ended up in this place, I couldn't rightly explain...... And I can completely understand that this sort of arrangement would not be for everyone!