The "Sushi Factor".
This is something that has come up in many relationships I've been in, observed, or heard about, so perhaps there is some value to be gleaned by putting it out there. In our house, we call it the "sushi factor". Here is how we discovered the S.F.: I'm a fan of sushi, and had invited S to go on several occasions. He pleaded off with, "I just can't stand fish, and raw fish particularly, so that's a no.". Fast forward some number of months, insert a new love interest he was seeing, and suddenly, there is S eating sushi with new person. Color me less than thrilled.
Now, with a "normal" mono relationship, you can go with the, "People change, and maybe they just like it now." theory when you see your ex, who was virulently opposed to xyz now engaged happily in it. Within poly, when you've made a request of partner, gotten a no, and then see them doing that with another partner, it can be a bit tougher to run with that theory.
Oddly enough, there seems to be strong anecdotal evidence that men are at higher risk of stepping in this particular mess. In every case I've directly observed, it's been the guy that said he wasn't interested in (fill in the blank), then proceeds to do it with new partner. Things like a shopping venue, type of food, activity, a sex act, or relationship style.
S and I were talking that over at lunch the other day, and his supposition, which may not be particularly flattering to our male audience, is that the desire to bag a new partner often trumps existing boundaries/preferences with current partners. Hunting instinct.
Now, there are a great many poly folks out there that will say that everyone is entitled to enjoy, or not, whatever types of activities and cuisine they wish to with whomever they want. They'd be right too. However, in practical applied terms, this may not be the best policy to run with.
Real life example from personal history: Back in the swinging days, I was out on a full swap evening with my partner at the time. Things were ramping up sexually, and the man I was playing with went down on me, and began to stimulate me anally as well. After a short time, I noticed that his partner seemed a bit upset, and took a break, grabbing her on the way out of the room to chat. She was embarrassed to admit it, but she was upset because she really enjoyed oral sex with anal stimulation, and asked for that several times, getting a no from her partner. What's the first thing he does out with someone new? Go down on her and stick his finger up her ass! Seriously, does anyone out there reading this think that's a winning strategy for domestic tranquility?
Now, some of you may be inclined to just write this off as NRE insanity, and while there may be an element of truth to that, NRE isn't an excuse for a lack of consideration or sensitivity. In the event that you are out with someone new (or in with them), and they bring up/you have this great idea you've heard before...somewhere...pause for a moment and consider where it originated. If it's with an existing partner, I respectfully suggest that it might be worth holding off on doing with the new person, and giving your existing partner first shot at it.
Now, am I unhappy that I can now sometimes cadge S into going to have sushi? No, I'll leverage that NRE-induced experimental bent for all it's worth! However, it would have been much nicer and possibly built more trust and intimacy in our relationship to have that level of willingness to try something extended my way without the need for NRE intoxication.
Note: This whole concept applies only to those things that everyone is willing to do/try. There is no suggestion that anyone should "take one for the team", or feel pressured to do something they don't want to do.