How close is too close?

I'm a firm believer that relationships should be natural. And they should. But what's natural can only be natural to the extent of the nature of each person in the relationship. If one of the persons in the relationship is an abusive alcoholic who absolutely needs his/her ass wiped after each dump, that would form, at least in part, what is natural in that relationship. The other part of what is natural is up to the other person. Is the other person an ass wiper or an ass kicker?

That makes a relationship seem a lot more combative than it needs to be, most of the time, but you get my drift. If I'm emotionally needy and the other person in the relationship is not prepared to give that much front-back-and-centre-plus-pretzels-knots-and-crosses-support, then the natural state of that relationship would be like that between a Hummer and quantity-controlled petrol station: unsatisfied for one, and defeating for the other.

That might be fun, too. But for how long?

In the other extreme, if two people are so close that there's nothing they don't know about each other, nothing they don't understand, nothing they can’t give, then wouldn’t the sex be like masturbating?