If you continue to have sex anally doesn't that make your anus stretch.

Not unless you damage your anal musculature. It stretches and reverts back otherwise.

That's what I thought too. It should be ok to 'exercise' your sphincter muscles unless you tear them, by not using lubrication or going to fast too soon. Then it's off to the emergency room. Would also be quite embarrasing explaining that to your parents and surgeons .