It's a long story I guess... and a bit convoluted. He gets turned on by the idea of me having sex with other men, in fantasy. He had a previous gf where he let this happen. He does say in reality he might just feel jealous but it (the fantasies) has definitely led to some amazing sex. He said me going and having a one night stand with someone does risk our relationship but he thinks it's worth the risk, once. (It also turns me on to think he might get jealous and channel that into a passionate bdm session too). From there we can only assess how we both feel. He wants to share the experience vicariously. For a few reasons he's not really big on one nighters, whereas I've had a few in my life (mostly craptacular to be honest, but they were when I was rather young). I know I don't just fall in love through sex, although it does make it happen faster if it is happening (as long as the sex is good anyway!)
He feels threatened by the idea of me being in love with someone else, that I'll end up leaving him because he's rather insecure about himself. Sex isn't so problematic for him where there are no feelings involved, he knows he doesn't own my body.
I don't want to railroad him into anything, and the fact we are talking about it means it's in his head now and he'll think about it occasionally, and challenge himself. He's like that. He understands me falling in love with someone else could be problematic. But I guess we'll cross that bridge if we come to it. I actually can't believe we've gotten this far...