The pre-nuptial agreement is becoming more and more common these days, and it's pretty much totally customisable, so I'd contend that we do (or at the very least, we can).
In a marriage that was not intended to make a man/wife pair, it's even more likely that such an agreement would exist, because there is little legal recourse for people wishing to leave such an arrangement.
What I am saying is that, even if you had this hypothetical marriage dealy with your best friend (male or female, doesn't matter for this example), and you weren't sexually involved ("I love X but I'm not in love with X" kind of thing, if this were a TV movie), it would still be a good idea to have some kind of mutual agreement regarding sexual behaviour. This is because sexual tension and jealousy are two of the most destructive human forces both in terms of the degree of damage that can be done, and the range of people they affect. The other examples you mentioned don't come anywhere close, and are more easily discussed or compromised on for the majority of people.