My male guppy is inserting its gonopedium into a cardinal tetra?

my male guppy is clearly under stress... because of a neon tetra which kept on biting the tail... now i transfered it to a 15 gallon tank with cardinal tetras and its inseminating its gonopedium into the cardinal tetra? why? is it gone crazy or just stress? and the cardinal tetra does not even have a hole there... except for the cr@p hole...