Something along the lines of:

MtF trans athletes don't have an advantage over cis women. If they did, you'd see them overrepresented on the podeum in the women's olympic events. So far, there is yet to be a trans 1st place woman. We've seen how far nations will go to win (Russia - Icarus doping scandal) so if it was the case that MtF have an advantage they'd be a lot more represented in the olympics. The olympics is the pinnacle of sports, and as such their rules are also to a very high standard. The rule is that MtF athletes must have been on HRT for two years to compete, to make sure they have no advantage.

They say: but they still have an advantage - bone length.

I say:

Well, firstly that 'advantage' isn't actually advantageous as indicated by the stats I just told you. Secondly, we see variance in the sizes of people all the time. Michael Phelps has a 7L lung capacity as opposed to the average 5L, freakishly flexible ankles and a massive wingspan, and hands. Will Skelton (rugby player) is 7ft something and has bones the thickness of tree trunks. Aleksandr Karelin dominated the wrestling scene because he was a genetic freak. Usain Bolt has more twitch muscle fibers than your average person. Yet we don't ban these people for their physiological variation.

They usually sit there a bit stumped.

I finish with: in fact, if we want to keep sports fair, one of the key things we need to do is to recognise trans people. Look at Mack Beggs. He's a wrestler who is mid transition to male, who asked to join the men's league in wrestling. His states wrestling federation didn't allow him. Now he suplexes women half his size in the women's league, ruining the competition really. All because people wouldn't recognize a trans man is a valid man.

The argument for FtM is a lot easier, it's just make sure their testosterone levels are the same as their competition.