'Fornicate' is a transitive verb requiring a direct object (and in this case, a preposition).
OK, It's not very nice to call Janet an 'object' but Tam Lin sure as hell shagged her.
But nobody said she was a witch . . .

Well up then spoke her father clear
and he spoke meek and mild
"Oh and alas Janet" he said
"I think you go with child."

Oh they will turn me in your arms
to a newt or a snake
But hold me tight and fear not,
I am your baby's father.