Given that your motive behind this policy is to find a minimally offensive compromise, it's worth noting that some people, especially trans people with "he" or "she" pronouns, will consider referring to them as "they" to be misgendering them and find it offensive. So your proposed policy fails at (and perhaps actively undermines) its *primary objective* of minimising offence caused through pronoun use. That, in itself, is probably a good reason to reject this idea.

But for the record, there are some other possible objections, too. Some are situational (in that they're uncontroversial points, but only apply in some scenarios); others are controversial.

1. Sometimes using "they" is grammatically ambiguous about whether it is standing in for a person or a plural noun. For example, if I say:

  > Bob climbed up the tower and put his bags on top. Then he fell off.

  then it is unambiguous that *Bob* fell. But if I say:

  > Bob climbed up the tower and put his bags on top. Then they fell off.

  then it sounds like the bags fell, rather than Bob.

2. The use of singular they, especially to refer to a particular named person with a known or presumed gender, is briefly confusing to many readers, especially non-native speakers, especially those who come from languages without a gender-neutral singular pronoun.

3. Some people consider singular "they" to be grammatically incorrect and object to its use on that basis.

4. Sometimes using gendered pronouns is useful for distinguishing between two characters without having to reuse their names. For example:

  > Jack and Jill went up the hill. He gave her an apple. She smiled and thanked him.

  Assuming that you know that "Jack" is a male name and "Jill" is a female name, you implicitly understand, when reading this sentence, that Jack gave the apple to Jill, and not the other way round. This is something you can only do with gendered pronouns. Sometimes writers (including me) will even deliberately assign different genders to two *hypothetical* people they are writing about, in order to allow them to be distinguished by pronoun. Enforcing singular "they" everywhere eliminates the opportunity to do this.

5. Using gendered pronouns is *normal* in English. This isn't in itself an argument against *you, personally, choosing* to avoid them, but is definitely an argument against *banning* their use, since:

  * Even if you try to ban them, new users who don't bother to read the CoC (or have poor English and *struggle to understand* the CoC, or read the CoC but then make their first post a year later after they've forgotten about it) will use gendered pronouns. Suddenly we've got to set loose the mods on a huge proportion of our new users to tell them off for... ordinary use of language that they didn't realise was against any rules.

  * Even users who are aware of the rule may slip up by accident.

  * Even most users who are aware of the rule would be likely to be uncomfortable and on edge in their writing, since they would be writing prose that feels unnatural to them and would know that simply slipping up and writing normal prose can put them in violation of the rules.