Ok I'll bite. People are born with physical advantages that make them better in sports than others, that is a fact people accept. That includes the fact that in many physical aspects, men are physically stronger, faster, etc than women. This is why they compete in separate categories. When it comes to a MtF competing in female sports, that is person who has developed a male body who is now competing in female sports. Yes, hormones will downplay these male aspects to an extent, but they will stay have an advantage.
I guess the moral question is whether this is fair or not. But I dont think someone is a transphobe or trying to be a dick when they say it is unfair.