I think it's the horniest thing a bloke could "EVER" experience, especially if you fancy the other girl and get to f**k her or get her to BJ you after. The sound of the "other" girl coming, and watching her face and body as she climaxes is AWESOME.
I feel extremely lucky that I have experienced this. I can see why people enjoy swinging!!!

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Sex is good.
But nothing beats the build up.