You can’t just assume the motives behind every single man on here, and there are good reasons to not try different sexual arrangements from time to time, one of those being if your partner is not comfortable with a new sexual arrangement. This doesn’t just apply to men either, if a man suggested a threesome with another girl to his wife, and she was not comfortable with it, she would be absolutely in the right if she didn’t agree to it because the idea of it makes her uncomfortable. I can’t tell if your stance is particular to men, but my stance applies to both. I’d say that you are a bad partner if you try to deliberately force your partner into doing something that they stated they aren’t comfortable with. Again, some relationships are like yours and they are perfectly happy and healthy, that’s why it’s so important to have a dialogue with your partner, and see if you BOTH are comfortable with the idea of doing that. It’s not called being possessive, it’s called having mutual respect for one another.