In Death Note, how far could "sexual" interaction go with a shinigami before it broke rule XXXVI?
I think that some of the meaning of the rule is lost in translation. Trying to translate the rule myself using my limited Japanese based of the wiki image I came down to (feel free to correct me if I am wrong)


  There is a male / female in the god of death, 
  Human book point descendants with humans are not allowed or impossible,
  The death god Same as above does not mate.


Which implicates OR rather then AND leaving the ambiguity. But also explicitly states descendants and not specifically sexual relationships. 

But lets assume the translators know better what they talk a bout then I do. In that case I would assume that the rule covers the aspect of interaction using genitalia being impossible, but other sexual relationships are also not allowed.

Lets first take a look at the definition of sexual relationships


  Sexual relationships: Interactions involving sexual behavior with another person, or other people including, kissing, petting, humping, making out, sex, anal sex, oral sex, rimming, t bagging, fucking, licking, hickeys, sucking. 


Reading through these, lets assume that relations which requiring the genitalia are impossible. All though this never is explicitly stated, the rule does some what insinuate this. 

This would leave us with things such as : Kissing, petting, humping, making out, licking etc.

A Shinigami does have a mouth, hands and perhaps a tongue. Quite a few of these sexual relationships remain possible to the Shinigami, without actual intercourse or chance of creating demi-god off-spring. 

So if we interpretation it in this way, it would make sense to cover the broader aspect of sexual relationships