The withholding of sex within marriage is considered to be an abandonment of the marriage.
Even so, if the wife, or husband, is withholding sex, divorce is a Biblical option. It is a willful breaking of themarriage covenant.
Sex outside of marriage is never an option.
I agree that it breaks the marital covenant. Each partner trades the potential for other sex partners for the assurance that the other partner will do everything he/she can to meet sexual needs.
Fair enough, until one partner decides he/she (usually she) doesn't want to have sex anymore, which means the other partner becomes celibate, which he did not sign up for.
However, I am a little iffy about it being appropriate scriptural grounds for divorce. I think Christ was pretty clear that adultery was the sole ground.
There are a bunch of things a spouse can do that "break the marital covenant." I am wary of elevating all of them to proper Biblical grounds for divorce.