My current GF has been curious about swinging and nonmonogamy in general, and not found anyone she could be open about it with. When we started talking, and she found that instead of the idea bothering me, I was an enthusiastic supporter, she was able to talk about some fantasies that she's never been able to say out loud before.
I don't think she really believed that it was okay with me. The first time she came to me after a hot date, she was so excited to shot me her creampie, and she came almost instantly when I pushed into her. I think that afternoon, she was finally able to relax and trust that I like her, even when she's leaking another guy's cum.
She's still at the early stages of the lifestyle, and I"m encouraging her to take small steps so that she's comfortable at each new experience. But so far, it's been a helluva ride.
To think that several guys let this one get away... well, it certainly works out to my benefit.
As to sloppy seconds (or thirds, or fifths, or whatever), the hotter and slippery-er (yes, that's a word now) she is, the better. Also, the more turned on she is the better. She gets SO turned on showing off for me, that the sex can't be anything but amazing.