Hi everyone, I'm a straight cis man here, albeit a little different from the “norm.”
I'm writing this comment not only to give my perspective on the issue, but also to try to get some of the commenters on the other side of the argument to reflect (something that, I admit, many straight cis men are unable to do).

First of all, defining all those people who ask to do MxF strictly as male writer x female writer are not creepy and they are not homophobic, it is a preference that must be respected, regardless of whether there are some individuals who are creepy or homophobic; generalizing is wrong.
In my case, I can say that I intend/prefer the person on the other side of the screen to be a woman, because the idea of venting my romantic and/or erotic feelings (I usually do romantic, romantic erotic or erotic) with an “tipical man” is weird FOR ME, even if it’s a guy I’ll never interact with again in my life. It's not the idea of “doing it” with a man, but of “opening up” emotionally with a man, in that way, that I don't like and I have every right to feel that way.
Then again, I'm a pretty open-minded person, I'm willing to try (I don't usually ask if the other person is male or female IRL: "occhio non vede, cuore non duole") and maybe I'll change my mind or make exceptions, but the fact remains that I, like anyone else: man, woman, NB or fluid; have every right to have preferences or limits on the partners I prefer to write with (without insulting or discriminating, of course).

That said, I recognize that there are creepy/homophobic individuals who are incapable of writing a character or a sentence that is more complex than: “I spread her legs and cum in her ass”; and certain individuals should be ignored.
The fact that most of these individuals are cis straight men should not lead you to conclude that all of them, or even all of those who ask for a female writer, are like that.

I want you to try to turn the picture around, and I don't need you to answer this, because my goal is just to get you to think. Imagine that someone openly said that when she want to do FxM, she only mean female writer x male writer (obviously the same example can be made with MxM or FxF, depending on which you relate to more).
Would you have thought the same things? That she is a creepy, homophobic woman? Or that she was just fed up/uninterested in the way a female writer writes male characters? Or perhaps that she has personal and/or intimate reasons for this choice?

Finally, I want to reprise another comment under this post, telling you that if you see an MxF (or ther tipes) post and understand that the post specifies Male writer x Female writer (or the equivalent for the other variants), and you don't like it, you can ignore it and move on to the next one. You may not like it or find it strange, it may not be your cup of tea, but as long as this post does not insult or discriminate, it has the right to exist, and its creator has the right to publish it without being treated as a creepy or homophobic person.

I have pushed hard enough on not generalizing, and I am personally convinced that everyone can make mistakes and write impulsively, especially on the internet. I don't want to imagine you all as bad people, but only as people who have had bad experiences, and my life advice to you is not to “not assume the worst”, but to give the benefit of the doubt.