"people" is plural, so you need "oppose", not "opposes". No comma between a subject and predicate. "biased" is a past participle acting as an adjective. Verb tenses don't enter into it.
People who oppose the pronoun 'they' can argue that 'they' has a root word 'he'; thus it is biased in that way.
I'm afraid I'm better at grammar than punctuation. I used single quotes in the example sentence because you did. Normally, I use double quotes. In all probability the single-quoted items in your sentence would be shown in italics in a printed context, but you may want to start another thread regarding the proper punctuation of that sentence, just to be sure. Someone else on the forum is bo