Well try to find out what his objections/reservations to becoming involved with other couples are. He might be afraid to lose you or it might harm your relationship in some way.

A good way to possibly introduce him to swinging would be to invite another woman to share him with. He may be thinking that you are only interested in it for your pleasure, but if you make it known that you would find it sexy to see him enjoying himself too, it might make a difference.

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Kitty's Lingerie
Kitty's 1000th
Surprise Threesome
Lazy Evening
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Once you can accept the universe as Matter expanding into Nothing, that is Something. After that, wearing stripes with plaid is easy. --unknown