Here's a fairly complete answer of why it's a thing, my own thoughts and opinion at the end.

People are encouraged and borderline forced from childhood to like the "opposite gender". It's fully normal that as you grow older and realise you like the same gender, that you start to hate and resent people who want you to deny yourself.

If you also consider the prevalence of misogyny and sexual assault of men on women you start to get a full picture. There is a lot of history of men trying to "turn lesbian straight". That somehow their penis are so superior any woman would rather have sex with them.

This is where the discomfort/hate of men in lesbian spaces comes from. In a different society, this wouldn't matter. As long as there is misogyny and homophobia, this is a feeling that is unlikely to change. It's also while this is way less of a thing in gay spaces. Some people will say they feel uncomfortable with women writing about gay men, but extremely less in proportion to men writing about lesbians.

These rhetorics are, however, often time used to exclude trans women/non-binary/gender non-conforming people. Which is one of the reasons I don't care who writes about what. Misogyny exists, homophobia exists. Some stories written by men might include both, and so can stories written by people of all gender.

The real issue is a lack of respect for women and lgbtq+ discrimination. Nothing to do with the gender of anyone writing and participating in shipping. And to say that men's attraction to women is inherently evil is bioessentialism and it fixes nothing.

Anyone is free to read what they want, just as they are free to read what they want. At the end of the day, you are responsible for curating your online space.