[You / your not loving me] is my nightmare.
Both forms of the personal pronoun are perfectly acceptable, though the genitive (possessive) form "your" is considered slightly more formal by some speakers.
Other than that, there is no semantic difference.
Note that "not loving me" is a verb phrase serving as predicate of the bracketed subject clause.
In a comment, Michael suggested 'jock'.
The following is from Lexico:
2 An enthusiastic male athlete or sports fan, especially one with few other interests.
but that does not exactly match the requirement of this question.
Under the synonym button, they have: sportsman, sportswoman, sportsperson