I had a crazy experience with my partner (now my ex), and my high school girlfriend...when I was in my 30s. I was living with my partner (we'll call her Ashley), and my high school gf (we'll call her Anne) was living in SF...so I hadn't seen her in years. I got an email from her, saying she was going to be in town, and that she'd love to have a drink. We stayed friends, so I thought it would be fun. I told my gf about it, and she asked if she could come. I didn't think anything of it, and we went to meet her when she got into town for a drink.
One drink turned into a lot of drinks, a night at a club, and the two of them really hitting it off. Ashley came up to me, and said that she wanted to bring Anne home with us. We'd had threesomes before (lots of fun) but I wasn't into it, because Anne and I still had lots of love for each other, and we had always had an intense friendship. I was worried Ashley would get jealous, and was against it.
She begged me to go for it, so I reluctantly agreed to it. However, by the time we actually got back to my place, it was me and 5 women...and ended up being a pretty crazy night, with lots of girl-on-girl/girl-on-me action...ending at about 6am.
After the other women left, Anne, Ashley and I didn't leave our apartment for 2 days. The amount of sex we had was staggering...it was non stop.
But, at one point, Ashley got up to go have a smoke. She went outside, and Anne decided that it was time to have me to herself for a bit. She jumped on top of me, and started riding away.
Then we see Ashley, looking through the window, with a glare that could have turned people to stone. She was absolutely NOT happy that Anne and I were having that much fun without her.
We talked about it, the three of us, and it seemed okay...until Anne left to go back to SF...and then Ashley went ballistic. I argued that I warned her what she was getting into, but she was so pissed that I could enjoy another women like that. But, this was the woman I lost my virginity to, and I explained AGAIN that I didn't agree to this...I was made to agree to this. She held on to that until we broke up (for other reasons).
So, just be careful. Even though it might be okay in theory...it's always complicated, because women are complicated. I would have done it again though, amazing fucking weekend.
For the record, I love threesomes. To be honest, I like it better than 1-on-1 sex. I have a HUGE libido, and one woman almost doesn't do it for me anymore. I got pretty spoiled with Ashley...she was really into threesomes (when it wasn't with my high school ex), and just being in a vanilla relationship gets sexually boring for me. I'm with someone I really love now, but sexually I still get bored and wish there was someone else there...which isn't cool for my current gf.