My parents have recently gotten divorced after 25 years of marriage.
That stinks that the marriage is terminated.
They waited until all us children had left home first. But it wasn't easy for us growing up in such a hostile environment.
They are allowed to marry another person now at least.
I think an affair is what caused the divorce.
Yes, at least there wasn't an annulment. Divorce is much different.
It sure is, the criteria for an annulment is a lot tighter than that of a divorce. 
Did they get divorced due to sexual incomatibility?
I'm sure it was a factor, only one of them was more sexually compatible with somebody outside the relationship.
Yea, thats a cause of divorce. But lack of independence is also one.