I've had a few experiences as a single woman.
It is really hard to meet spontaneously and move to that level!
I used a site and had a nice afternoon with two individual people from a website in a hotel (ffm)
However the guy really struggled with stamina for two women, he was a bit older than us and his recovery rate meant that he didn't have PIV with both of us.
I tended to forget the guy tbh, as it was the female experience I enjoyed and that's hard to come by.
There is something amazing about kissing and tasting another woman.
But yes agree you need to be very secure if in a relationship.
I'd do it again but Im not sure I'd want my dp to have PIV with another woman as we have deep feelings for each other, but we had a threesome early in in the relationship as FWB.
We were fine and at the time it was a huge turn on to watch him make her howl 
But it was the choice of woman in the ffm. She got really competitive on the evening so it got a little bit embarrassing. I'd told her things I hadn't or didn't do with him and she made it her mission to complete them, asking who was better at certain things etc. (she got very drunk).
The next day she asked if I'd mind her meeting him alone for some time but the evening before he left he'd said he wasn't interested in her and just enjoyed time with me. (Actually hard to believe but it was that night that changed the whole basis of who we were when we realised we had something of a chemistry)
I found out that she'd contacted him before asking me, and was just hanging around to see if he'd say yes, come over. I felt a bit pissed off about that, it was underhand.
From her side though I guess the evening of us both discovering being so into each other must have been a bit hard on her, but we were both very polite and gave her loads of attention and pleasure. However it was something that we really couldn't hide.
I think if I wanted the experience again (and it would be mine, he's happy as we are) I'd suggest we go to a club for a ONS experience. I don't want a relationship or friendship with a woman. Just an experience.