I've been in that situation... and I did it. I had talked a lot with my S/O about her and another girl getting it on and she was game, but mainly going to do it 'cause I wanted to see it. When she countered my proposal with her own, I figured it was fair enough. Although she got her proposal fulfilled first. Just don't ask me to kiss another guy...

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Put me on wheels and I'll turn tricks.

Clever? Nah, I ran out of that years ago. But if you find this, let me know, k?
"The road goes ever on..." ~ Tolkien

In memory of my friend skip...
Go then, there are other worlds than these