I'm an adult (38), financially independent and have been living far from my parents for years. I go home maybe 2-3 times a year for a visit. My parents are still married. I'm very close to my father and can be pretty open with him. With my mother, the relationship is a rockier because she's very anxious and judgmental. For example, I no longer tell her when I travel because she needs to know who is taking me to the airport, am I at the airport, am I in the plane, did I land, who's picking me up, etc. So over the years, I've stopped telling her what I'm doing.

When I call home, my mother constantly complains to me about my father. In general, I try to be sympathetic and just let her vent, even if it's annoying or upsetting to listen to, because I'm literally the only person she feels comfortable venting about my father to. To everyone else, she tries to show that she has this perfect marriage. However, sometimes I get really sick of it. We've fought about it many times. During my busier periods, two or three weeks might pass without us talking--yet the moment I call, she just launches right back into how my father is driving her crazy.

I tell her that they should do marriage counseling: she gets angry and says she'd be "embarrassed" to tell a counselor about my father's behavior.
I tell her that I don't want to hear about my father: she says she isn't talking about him, she's just telling me about her day because this is what happened and "well you asked me how I am so I'm telling you."
I tell her that it's inappropriate for her to complain to me about him: she accuses me of talking about her with him (okay we do a little but not that much) and also that it's important to her that I "understand what's happening in the house."
I've also snapped at her a few times to get a divorce or separation: she gets REALLY mad and tells me that I'm selfish and I don't understand what it means to be committed to another person and love someone for decades. Of course, then I yell at her that if she doesn't think I understand, stop bothering me with her marriage problems, which brings her back to the "I need you to understand what's happening here in the house."

Nothing is "happening" in the house. Before the pandemic, a "crisis" might happen because my father had Chinese food for lunch and she'd be super upset that he's going to give himself a heart attack.

Today's fight--I finally hung up on her because I couldn't listen anymore--was about how he wanted to try preserving an avocado in the freezer by mashing it with lemon and putting it in plastic, and that she never mixes lemon and plastic because it's a health hazard. Apparently he totally exploded, said he was done cooking in the house for all time, and went downstairs to sulk. I know why he exploded--my mom is really paranoid about toxic things, like to an extreme level, and it's driving my father crazy.

I'm an adult so maybe I should suck it up and just let my mom vent if that's what she needs. But there are days I can't listen to it. Also, I can't validate her, if that's what she wants, because I just don't agree with her point of view. Advice? Is it wrong for parents to complain about the other parents to their kids, even if the "kid" is already an adult? Is it selfish if I refuse to listen to her venting when I'm the only person she's comfortable venting with?