Yes please would love to hear updates on how you get on with it 
This depends on the type of joists you have, and their width. I think @Lexifer’s design is an excellent suggestion for many situations, but not all. If you have a modern lightweight truss roof, for example, the joist is a critical structural component, and in some cases it‘s pretty narrow to save cost and weight. Drilling a large hole vertically through this type of joist for a bolt, could in some situations weaken the individual roof truss significantly 
I‘d say it‘s safer to drill through a beam placed on top, with a large washer or square plate beneath the nut.
Alternatively, the joist you’re aiming for can be reinforced along it’s length to strengthen it and reduce flexing, by bolting/ screwing extra joists either side or along one side.
From my experiences of climbing: in suspension, the more points of contact, the better, for comfort and endurance! 
The cheaper over-the-door type swings are good for an athletic quickie, and to assist a partner who wants to lift you up for sex against the wall. But it is a workout!
If you want more of a feeling of weightlessness, and relaxation, I really recommend a proper ceiling mounted swing, and even adding your own adjustable straps for support. They‘re so lush once they‘re set up to support everything 
The chains will be fine! This type of product must be designed to be fit for the purpose, and a safe loading weight is quoted to conform with it‘s CE marking.
The Dominix Deluxe swing has a limit of 150Kg, for example.
Hope your swing dreams all come true!
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Would love to try the sex swing I heard from my wife’s friend that they have got a door hanging one and she said u should go all out if ur interested in getting one