The Harkness Test is a good start if you live in a soft-sci-fi universe. But it's not perfect: some of the terms used are vague (not even just the question of "≥ Human Intelligence"), and it only defines what is acceptable to human ethics, not alien ones.
A simple example: Suppose that the alien is a sentient AI. Its reproductive process is as simple as "Insert USB stick, Ctrl-C, Ctrl-V, eject". Even an AI that knows nothing about the world - that is at a stage of life equivalent to a newborn baby - would be "sexually mature" by the most reasonable definition. But that does not mean that it is emotionally mature or capable of fully understanding the consequences of its actions. It passes the Harkness test, but it really shouldn't.
Sex is
complicated. Incredibly moreso when you throw aliens into the mix. There's no guarantee that sex is even tangentially related to childbirth, let alone romance or (god forbid) happiness. Just think about how flowering plants reproduce, and imagine a sentient version of that. It's not even guaranteed that the alien is naturally capable of reproduction - maybe they're grown in vats, maybe only the queen lays eggs, or maybe it's a single entity that has existed since the dawn of life, never developing any reproductive capabilities because it never needed them. I know the Harkness Test only serves to narrow the field from "sex is possible and desirable" to "sex is appropriate", but there are still far too many additional variables to consider.
Those for whom the words "thri-keen erotica" mean nothing will want to read
the discussion thread on comic #74.
The webcomic
Spacetrawler features a scanner for "Sexual compatibility" that gives a rating between 0% and 100%. I like to think that its designer encountered this sort of problem before, and determined to fix it. The closest match in the comic -
94%, between a human and a fahldwian - certainly doesn't seem to be based on physical appearance.
... in bed.