If I was dating a woman I truly cared about and she was okay with a threesome with another woman, I wouldn't do it. It's been shown time and time again that the vast majority of the time it ruins things. Jealousy is a real human emotion and it's very common.
I was dating a "pansexual" chick once who once asked what my fantasy was, so I told her it was that. She said she was down... Then, every time she was mad at me, she'd bring up that I wanted a threesome and how it was because I didn't want to be with her etc.. And that happened when we hadn't even looked for a woman to join us. I can't imagine what would've happened if we actually had one
Now if it was just a chick I was banging who wanted one, I totally would.
And that isn't even counting that the majority of poly people manipulate their partner so they can get their genitals wet with another person.