Joe and Nicky’s love is legitimately beyond measure and they’re extremely secure in it after so many centuries.
This is why, even though 99% of the time they don’t want anyone but each other, if one of them wants to casually remark on how attractive someone they run into on the street is, or enjoy flirting with someone for the little thrill of knowing that they’ve still got it even after so many years and can still attract someone, or even—very rarely—have a one-night stand with someone, it’s no problem as long as it’s fun and consensual. They even tell each other everything about it without shame or judgment.
There’s no need for any jealousy whatsoever because Nicky and Joe know that nothing in the world could even come close to approximating what they have.
Joe/Nicky, consensual infidelity
This is why, even though 99% of the time they don’t want anyone but each other, if one of them wants to casually remark on how attractive someone they run into on the street is, or enjoy flirting with someone for the little thrill of knowing that they’ve still got it even after so many years and can still attract someone, or even—very rarely—have a one-night stand with someone, it’s no problem as long as it’s fun and consensual. They even tell each other everything about it without shame or judgment.
There’s no need for any jealousy whatsoever because Nicky and Joe know that nothing in the world could even come close to approximating what they have.