When you get in on a couple who want a third guy in the mix?

I kinda always wanted to d-ped a woman with another guy, but I don't date women or ever have that would ever consider something like this.

I'd also probably not want to do it with a girl I'm seeing.

For some reason I think an intimate couple that want to experiment with another guy, is that cheating? I mean, technically it's not like you're going on a date with another woman, going to her place and ****ing, it's basically a couple ****ing and need a hand.