The desire for novelty comes from bored or unfulfilled from your current relationship, which would contest the "security" of the relationship if it is impossible to live without it.
That I am clearly fulfilled with my existing diet doesn't stop me from trying new things on the menu, nor new restaurants with entirely new menus.
And the desire to try new things doesn't imply that the absence of them is "impossible to live without" — even back out of metaphor land and back to human sexuality, lot of gay people used to live straight lives.
Which is my point: the relationship isn't "secure" if you're to find out that you're actually gay.
Are you unfamiliar with the concept of a metaphor? I did explicitly note that food was a metaphor. After all, your original comment said:
> if it is impossible to live without it.
Which is a much higher standard than you're now using, one where food is a useful metaphor as one does actually need it to live, albeit as a category and not any specific food — I like capsaicin, alliums, and fresh coriander, and yet I can live without them (e.g. if I had a partner who was allergic).
And are you unaware of the history (and indeed current diversity) of rules about sexuality, including the cases where couples aren't/weren't allowed to separate and gay relationships are/were forbidden? You can know full well what you like, and still be bound unfulfilled but alive. Thus, that question of fulfilment is still present when your relationship is not merely "secure", but the security is enforced by law.
What does "secure in their relationship" mean to you?
Prisoners are secure. They are also, to use the word you used earlier, bored.
Don't mistake "secure" for "fun". It is an independent axis of consideration.
Lots of people value security over fun, including when it comes to sexuality.
Not everyone, but lots of people.
And going further up this thread back to what Quinzel wrote:
> If I felt secure in a relationship, having a partner that wants to experiment with others/date others wouldn’t really bother me.
Alice can feel secure about Bob always being there for Alice when Alice needs it, while also being comfortable with the idea that Bob sometimes boinks Chris, and this can be either true or false, this is independent of if Alice wants to boink Dee.
Telling somebody you don't know in a situation that you aren't familiar with that they aren't secure in their relationship comes off as ignorant at best.
Especially since OP describes something on the more casual side of the ENM spectrum as opposed to full fledged polyamory.
There's stuff my wife just doesn't like to do! Like... hiking. Yes, hiking is what I look for in other partners.