Do you think its true that when it comes to relationship once a cheater always a cheater? Why or why not?

Yes, but I see that more of a positive than a negative

Nah.
Some people cheat and they realise it's a mistake.
Some people cheat and its their nature. Those probably do not change.

I don't always think so... Some people have a non monogamous mindset.
So effectively until they come to that realisation and live their life that way they will continue to hurt people.
There are others that live for the thrill and excitement of cheating, stealing, lieing...
Others make mistakes and succumb to weakness...
I think we're all allowed at least 1 mistake.

I've also said under the right circumstances, anyone will cheat. No more true than your teacher's statement.

Blanket statements are rarely right. They might indicate a high probability for X occurring, but certainly not a 100% certainty.

Once a cheater probably still a cheater is a fairer statement.One rule fits all can be harsh.Our life experiences change us and we learn from them,well most of us anyway.

I would say it just means that person is capable of that behavior under certain circumstances, not that they’re guaranteed to do it.

Cheating is a taboo in a relationship so once one person cheats it becomes far easier to do it again.

No. I have known several people who were cheaters, but became committed to one person and never cheated again.

Depends. Like anything else people are cable of change and growth.

I think so. Some people just need to feed their ego.

It's been my experience yes.

Probably, yes.

No.

No blanket statement is 100% true all of the time but if a person has cheated on the past it is a good indicator that they may do it again.
Also once you do something once it becomes easier to continue doing it.

Yes