I've recently joined a club that has a mixed doubles league. I'm surprised to see how aggressive the men are when poaching. I'm not just talking about intercepting the odd put away shots but even run-of-the mill dinks or returns of serve. They practically push their partners off the court and play by themselves!
My inclination is to say "back off!" but thought I should check in with you first. What's going on?
While pickleball has garnered a reputation for being friendly and social, it is not immune to the mistaken beliefs that permeate all parts of society. That is, the sexism that we see in daily life doesn't magically disappear once people walk on the court. In doubles (mixed or same gendered), a player poaches a ball because they think they can do something better with it than can their partner. This could either be because they are in a better position to hit a particular shot or because they think they are simply a superior player. Given the extent to which you are seeing this happen in mixed, it sounds like you're playing with a group of guys that have two thoughts running through their mind: 1) I really want to win; 2) I'm better than my partner so I should take most of the shots.
This can be an extension of the common -- and foolish -- attitude in mixed doubles of "hit to the woman". Smart doubles teams hit to the weaker player, regardless of their gender. If it happens to be the female you decide to pick on, so be it. But if you want to have success on the court, this decision should be based on assessing your opponent's skill and performance, not gender.
As for how you should proceed? I think you are totally within your rights to ask your partner to ease off on the poaching. Let him know that while you're there to compete, it's actually not that fun for you to watch him take all your shots. Your position will be strengthened when you show him that he doesn't need to worry about you pulling your own weight as a teammate. Execute great shots and he'll be more than happy to cover his side of the court and do a little less running.