One thing worth settling first. You're not trying to win this.
Go in looking for a yes and you'll get either a row or a yes that doesn't survive contact with an actual evening. You're asking a question, and questions have more than one permitted answer.
Pick the moment badly and the rest doesn't matter
The most common mistake is raising it in bed. It feels like the natural moment because it's the only time the subject seems to fit. It's the worst one.
In bed, the question arrives attached to arousal, and whatever your partner says is shaped by that. Agreement given then often isn't agreement at all, and disagreement can feel like a rejection of the whole evening. Either way you learn nothing reliable.
Pick somewhere ordinary instead. A walk, a drive, the kitchen on a weeknight. Somewhere neither of you is naked and either of you can change the subject without it being a statement.
Give it room, too. Not ten minutes before you leave the house, and not after a bottle of wine, when the conversation is easy but the memory of it is unreliable.
What to actually say
Three things, in this order.
What you're asking. Be specific, because vagueness gets filled in with the worst possible interpretation. "I've thought about us having a threesome" is clearer than "I've been wondering about opening things up", which could mean almost anything.
Why, honestly. This is the part people skip, and the part your partner needs most. Not why in the selling sense. Why as in: what the appeal actually is for you.
If it's the idea of watching them enjoy themselves, say so. If it's plain curiosity, say that instead. The vaguer you are, the more it sounds like there's a reason you're holding back.
That they can say no. Out loud, in words, unprompted. "I wanted to ask rather than wonder about it. If it's a no, that's completely fine and I won't bring it up again." Say it and then mean it, because you're going to be held to it.
Then stop talking. The instinct is to fill the silence with reassurance, and what comes out is usually a second round of pitching. Let them think.
The thing they're probably thinking
Almost everyone's first reaction is some version of: am I not enough?
It's worth answering before they ask, because a lot of people won't ask. They'll just sit with it. Something as plain as "this isn't me being unhappy with us, and if it costs us anything I don't want it" does more work than explaining what you'd like to happen.
If you can't say that honestly, and this is actually about something missing, then the threesome isn't the conversation you need to be having.
If the answer is no
Then it's no, and the useful thing is to make that cost them nothing.
Don't sulk, don't go quiet for two days, and don't raise it again in a month with a new angle. A question you keep asking stops being a question. Your partner learns that no doesn't hold, and after that you won't get a straight answer about anything.
Some people come back to it themselves, months later, having thought it over in their own time. That only happens if the first no was received properly.
If it doesn't come back, you have your answer, and you got it early and cheaply.
If the answer is "maybe"
Maybe is usually honest, and it's the answer most couples actually get.
It normally means one of three things. They need time. They need more detail before they can picture it. Or they're fine with some version of this, but not the one in your head.
All three are workable. None of them is a yes yet.
The useful next move is to find out which. Ask what would make it a no, rather than what would make it a yes. The answer to that question is where the real terms live, and it's far more informative than enthusiasm.
If the answer is yes
Take it slowly. The gap between agreeing in principle and being comfortable on the night is where most of this goes wrong. That gap closes with conversation, not with momentum.
Before anyone goes near a profile, work out what you each actually want, what's not happening, and how either of you stops things without a debate. That's the substance of it, and it deserves its own conversation. There's more in rules, boundaries and jealousy .
One more thing to settle while you're there. Whoever joins you gets the same right to call a halt, whenever they like, with no reason given. If that changes how appealing the idea feels, better to find that out now.
When you're both ready to see who's around, set up a free profile and look before you commit to anything. Joining and browsing cost nothing.
If you're the one who was asked
You don't owe an answer today. "I need to think" is a complete sentence and a reasonable one.
A few things help. Separate the question from the person asking. Wanting this doesn't mean they're dissatisfied with you, and it usually isn't a coded complaint.
Ask what the appeal is for them, plainly. And notice how they take a no, because that tells you more about the relationship than the question did.
If you're a yes, be a yes about something specific rather than in general. Vague agreement is how people end up somewhere they didn't intend to be, on a night when it's hard to say so.
Questions people ask
Is it ever okay to ask your partner for a threesome?
Yes, provided you're genuinely asking. What isn't okay is asking over and over, asking in a way that makes saying no expensive, or presenting it as already decided.
How do I bring it up without them thinking I'm unhappy?
Say that part first, before they have time to wonder. Most people's opening interpretation is that they're not enough, so address it unprompted rather than waiting to be accused of it.
What if they say yes but I don't think they mean it?
Then treat it as a maybe. A yes given to end a conversation holds right up until the evening itself. Ask what would make it a no; if there's no answer to that, they haven't thought it through yet.
Should we agree rules before or after we decide?
After you've agreed in principle, and well before anyone messages anybody. Rules worked out in advance are the ones that hold when it matters.
Is there a rule for how couples should handle this?
Not a useful one. The internet is full of numbered relationship rules, and none of them survives contact with two specific people. What works is saying the awkward part out loud, early.
Where do we actually look?
A site is one of five realistic routes, and not always the best one for where you live. All five compared is the honest version.
What if we agree and then can't find anyone?
That's the usual experience, at least at first. Most couples underestimate how long it takes, and how much rests on how they write about themselves. What couples get wrong looking for a third covers it.