I suppose, then, if you want to sort this out on your own, you just need to ask yourself a couple of questions:

Do you believe what your partner told you last time?
Do you think that your partner is loyal and trustworthy?
Do you trust your partner?

The thing is, anything can happen, at any time, to any relationship. No-one can really stop themselves falling in or out of love.

Your partner has indicated that they see their future with you.

These other 'friends' presumably know that.
But still they flirt.
But still they are not with your 'SO' ~ you are.

Quote:
... from my SO's standpoint. We're together, and will continue to be until one or both of us decides we're not

You know what you see in this person ~ so you must understand why others see it too.

Hopefully these other 'friends' are not being led on ~ they must realise that you are 'a couple'.

Let them be jealous of you. You are the one in the relationship. Forget them. Let them have their dreams.

If, by some chance, one day, one of them does settle down with your partner, then there is not much you can do about it. You would be better off ensuring that this does not happen, by keeping your relationship happy and fresh.

I still think that honesty and communication are healthy, though, and I still think that, if you find that you are becoming unhappy and annoyed, then you should explain this to your parter.
No-one can deal with something that they are ignorant of, so give your partner a chance to help you accept and understand things.

Quote:
'One of these people gave my SO a very thoughtful and expensive gift last year. It took some thought, research, and I'm sure they had to save money to make it happen. There was nothing wrong with the gift, but I felt the whole detailed gesture was a bit out of place'

From a purely personal point of view, I, too, would feel wary if a girl gave my husband an expensive and 'thoughtful' gift. I wouldn't consider it appropriate (unless she were very rich and dished out gifts to one and all) and I think that I would be very concerned that he made it clear to her what their relationship was - and what it wasn't. So you are not the only person who would respond like this ~ and I don't think that it is just jealousy.

What would happen, if it were the other way around, do you think?

"The secret of success is constancy to purpose" - Benjamin Disraeli.