What if only one partner wants to open the relationship?
Tread very carefully. Asymmetric desire for ENM is the single most common failure pattern in opening relationships. Healthy paths exist but they require both partners genuinely wanting the structure, not one wanting it and the other tolerating it under pressure.
The data and the clinical writing are unusually consistent on this point: when one partner wants to open a relationship and the other doesn't, opening it anyway almost always damages the relationship rather than meeting the underlying need. The asymmetry doesn't resolve itself once the structure changes; it just gains a new surface to play out on.
What looks like 'agreeing' can in fact be: coerced acquiescence (agreeing because saying no costs too much), trial-and-hope (agreeing in hopes the desire will dissipate), conditional agreement (agreeing while privately planning to ration in a way that makes the structure unworkable), or genuine ambivalence (truly not knowing, mistaking that for consent). None of these are stable platforms.
Healthier patterns when desire is asymmetric: a sustained period of conversation, possibly with a specialised therapist, about what the wanting partner is wanting and why. Often it turns out to be specific — a particular kind of connection, a specific unmet need in the existing relationship, a desire that doesn't actually map to opening so much as to changing something inside the existing partnership. Sometimes it turns out to be foundational — the wanting partner is polyamorous by orientation and the relationship was monogamous by mismatch. The latter is hard but at least clear.
Where the conversation tends to land: either the underlying need can be addressed inside the existing structure (sometimes through changes to the relationship's emotional shape, sometimes through specific permissions that fall short of full ENM), or it can't, in which case the partnership has a serious mismatch and the partners have to decide whether to address it directly. The decision-making is hard. What does not work is opening the relationship on shaky consent and hoping the structure produces the consent retroactively.
A useful resource here is the book Opening Up by Tristan Taormino; another is Polysecure by Jessica Fern. Neither will tell you what to do but both can help structure the conversation.