One partner wanting to open up an existing monogamous relationship is one of the most common and most difficult entry points into CNM. If you're the one who didn't raise it, you're likely somewhere between blindsided and trying to be fair to a partner you care about while also being honest about what you actually want.
What's actually being asked
The first thing worth getting clear is what specifically your partner is asking for. "Opening up" covers a wide range of things: occasional sex with others while maintaining your primary bond, pursuing independent romantic connections, a particular relationship structure they've been thinking about, or sometimes something more vague, a desire for something different that they haven't fully articulated yet.
Understanding what they're actually asking for versus what you might be imagining they're asking for is important before you respond. "Can we open up?" is not the same as "I want to have sex with someone specific" or "I want to maintain a full secondary relationship," though any of these might be underneath it.
The pressure to respond immediately
This conversation often comes as a surprise, and there's an impulse, in both directions, to resolve it quickly. Your partner wants to know if this is possible; you want to know where you stand. Neither of you benefits from a response given before you've had time to think.
Taking time to consider is not a betrayal of your partner. "I need some time to think about what this means and what I actually feel before I respond" is a reasonable and mature position. Decisions made under the pressure of an emotionally intense conversation in which you've just received unexpected information tend to be less considered than decisions made with some time and space.
Questions worth sitting with
Before you've decided anything, there are questions worth thinking through honestly:
Is this something you could genuinely explore, or does the idea produce such strong discomfort that you know already it's not for you? The answer to this doesn't need to be immediate, but it's worth being honest with yourself rather than trying to talk yourself into or out of it.
Are there specific aspects of what your partner described that feel more or less acceptable? CNM isn't a single thing; the specific configuration matters a lot. Some people are comfortable with sex-only arrangements but not romantic connection; others find the reverse true. Understanding your own response in more detail is useful.
What would need to be true for this to work for you, if anything? Are there structures, agreements, or conditions under which you could imagine this working? Or does the whole concept feel incompatible with what you want from a relationship?
The consent question
CNM only works with genuine consent from everyone involved. Consent that comes from fear of losing the relationship, desire to be the "good" partner, or from not believing you have a real choice is not genuine consent. It tends to produce outcomes that are bad for everyone, including your partner.
If you agree to open up while actually hoping it won't happen, or while feeling that you're sacrificing something fundamental about what you want from a relationship, the underlying conflict doesn't go away. It tends to surface in how you respond to the reality of your partner's connections, in ongoing resentment, or in gradual breakdown of the arrangement.
Saying "I don't think this is something I can genuinely consent to" is information your partner needs. It's a harder conversation than agreeing, but it's an honest one. What happens next, including whether the relationship continues, is a separate question.
If you want to try
If after genuine consideration you're open to exploring, the practical starting points are: slowing down enough to have several conversations rather than one, agreeing on what you're actually trying before either of you pursues anything new, and building in explicit checkpoints to assess how it's going.
People who enter CNM slowly, with explicit agreements and regular honest communication about how they're actually experiencing it, tend to fare better than people who rush into practice before the conceptual groundwork is established.
If you don't want to
"I've thought about it and I don't want this" is a complete answer. You're not obligated to want non-monogamy because your partner does. The incompatibility this creates is real and doesn't go away by either of you pretending otherwise.
What happens to the relationship when one person wants CNM and the other genuinely doesn't is one of the harder realities of this situation, and there isn't a clean answer. Some couples find genuine compromise (structures neither had imagined that both can live with); many don't. Pretending the incompatibility doesn't exist tends to defer rather than resolve the difficult decision.