The Oxford English Dictionary defines sex as “physical activity between two people in which they touch each other’s sexual organs, and which may include sexual intercourse”... seeing that the koi, despite being hapless, is not a people, and it doesn't sound like the pervert touched the koi's sexual organs, I'm going to have to say that they did not have sex.
Also, is 51 considered a senior citizen in Thailand?
A koi is ok, but if you want the best living fleshlight you've gotta get a rattlesnake. Just be sure you have the fangs removed first, and then use it before the next set of fangs come in, otherwise that's a VERY embarrassing trip to the ER.