I've noticed this with two partners I've had and it's their reluctance to let me go out with my friends on my own.
For example, I'd rather she wasn't with me and my friends when I have a few beers. I see that time as where I can be an idiot and not have to consider my actions towards my partners.
Another, seeing an old friend and catching up. Why does she feel the need to be there too? And be angry about exclusion.
So, other females, do you respect your partners need for down time, or do you think I'm in the wrong and should include her with most activities?