a high fiber diet has nothing to do with that at all. fiber helps your digestion and keeps you regular- that is not connected with tightness of an anus.
an honest response- i'm not emotionally invested in anal sex, so here is an honest answer that you won't see from those men who worship the idol of anal sex. If you aggressively fuck an anus, yes, it will loose its muscle tone over time. i'm not sure about the exercises- it would be an awkward exercise to say the least. If you have a gay physician, you could ask him. But exercise or no exercise, stretching out your anus will occur with repeated, over years, of anal penetration, especially if you use large dildos, butt plugs, and other anal toys. sometimes it stops tightening. the same thing occurs if you 'crack' your neck a lot. eventually you stretch the muscle and joint too much and your neck will crack as soon as you move it, with painful consequences. why would you want this to happen to your anus? it won't happen to most bottoms, but those who are 'pig' bottoms or 'power' bottoms will face this fate on day. how nice to have to wear an adult diaper because your shit will leak out. not for me, no thanks.