The second invitation is the difficult one.

We talk about 'the first time' constantly. Nobody mentions how weird a SECOND invitation could feel.

Because once feels like a story. Twice sounds like you are deciding something about your life.

Or maybe people had a nice time and want to repeat it?

Could be. But imagine only one of the couple wants to.

Or the guest assumes he's now a regular part of the picture.

I'd hate having to explain that once doesn't mean access forever.

Then explain it before you have to. I wouldn't want somebody to be guessing.

What if my partner wants to meet the same person again and I feel more uncomfortable the second time?

Then the second time doesn't happen just because the first did.

Right. Nobody has to repeat an experiment to prove they meant the first yes.

I keep thinking the third person should get an honest answer too. Being left hanging sucks.

Absolutely. 'We enjoyed meeting you, we aren't arranging another evening' is a complete sentence.

How do you say 'maybe later' without it sounding like a promise?

You don't, unless you actually mean it. 'We're not making plans' works fine.

I hate how sensible you all suddenly are.

We can go back to being ridiculous after lunch.