I am a 30-year-old straight man who has always known that he is a poly. The woman I love is not a poly. She is a monogamous person. When we started being sexual, it was a strictly friends-with-benefits arrangement, although a sexually exclusive one, at her insistence, and I agreed to that because neither of us expected anything long-term to come of it. But we fell in love, and now I can't imagine life without her. She is amazing, and I love her like I've never loved any other woman. But she has asked me to betray my sexual identity by remaining sexually exclusive. If I cannot commit to that, she does not want to be with me. I am not asking the same of her: She does not have to sleep with other people to keep me in her life. She is, however, insisting that I not sleep with other people to keep her in my life. Can someone who is poly be happy with someone who isn't?
You are not "a poly."
Poly is not a sexual identity, PP, it's not a sexual orientation. It's not something you are, it's something you do. There's no such thing as a person who is "a poly," just as there's no such thing a person who is "a monogamous." Polyamorous and monogamous are adjectives, not nouns. There are only people — gay, straight, bi — and some people are in monogamous relationships, some are in open relationships, some are in polyamorous relationships, some are in monogamish relationships, some are in four-star-general relationships. These are relationship models, PP, not sexual identities.
So the question isn't "Can a poly be happy with a monogamous?" The question is can you, despite your clear preference for nonmonogamous relationship models, be happy in this relationship? Do you love your girlfriend so much that you're willing to pay the price of admission that she's demanding — you're willing to behave monogamously (adverb!) — in order to be with her? Yes or no?
Since your girlfriend has already indicated that she's not willing to have a nonmonogamous relationship with you (or anyone else), PP, the choice is yours to make. If you truly can't live without her, if she's the-one-you're-going-to-round-the-fuck-up-to-the-one, you'll have to be monogamous. If that's not something you're willing or able to do — and "willing" and "able" are two different criteria, and you'll need to make an honest self-assessment on both counts — then end this relationship and go find someone whose romantic desires more closely align with your own.
I am a lesbian-identified bi woman who has been with my ladyfriend (also a LIBW) for seven years. She recently brought up her desire to have a threesome. I've had a handful of group-sex experiences, and I know that they can be fun but they can also go very wrong. I am worried that she isn't prepared to see me have sex with a man, and I fear that once we are in the moment she won't be assertive enough to stop something that she may have agreed to beforehand but suddenly isn't comfortable with. What is the best way to test the waters?
Our next concern is who to invite into our bed. We would prefer it to be someone we wouldn't have to see again, so friends are out. However, I am concerned about just finding a random person on CL or Adult Friend Finder because, being in a lesbian relationship, we definitely have run across men who think we "just need the right penis." Basically, I want a man who I know is friendly with the queer community and will respect our relationship and our boundaries. Where do we look for this?
Another Bi Woman
Established couples that want safety, respect and a measure of accountability from their very special guest sex stars, ABW, should look first to flirty friends and friendly exes. But you two, like so many threesome-seeking couples, want the perfect person to materialize immediately before sex and disappear immediately after. That means finding and vetting a stranger. And online personal ads are the best way to accomplish that. State in your profile that you're looking for someone who (1) is queer-friendly, (2) respects your relationship, and (3) doesn't think the "right" penis will turn you both straight.
Some guys will tell you whatever you want to hear, of course, which means you could wind up in bed with a man who doesn't believe any of those things. But he'll know to keep his mouth shut, ABW, and since you're not going to see him ever again, does it really matter what he thinks?
As for your fear that your girlfriend won't speak up in the moment: Address that with her, address it at length, and consider taking penis-in-either-of-your-vaginas sex off the menu for your first threesome.