Fitness and dedication are both really attractive qualities, but so are flexibility and spontaneity. Plus, getting rejected is never fun, and if the reasoning isn't crystal clear it can feel like an insult even when that isn't intended. Not a lot of guys (as far as I can tell anyway) would choose the gym over an offer for sex, so girls will probably be surprised by that and might think you're not really interested in them. It depends on the situation to an extent, but I would say it's more likely to be a turn off.
It can depend on your relationship with the person. For example, if you are in a serious relationship with someone, live with them, and have sex frequently, it's very justified to have to turn down sex at a given time to make room for daily activities and other obligations. But if it's a new thing, rejection is going to be a bigger deal.
if this girl is not your girlfriend and you have no interest in her, it is not a turn off.
if she is your girlfriend it would be.
Turning people down when they want to copulate with you is always a turn-off regardless of reasoning.
What would you do?
Play it off as if it's for her. If you ever turn someone down if you explain that they benefit from your advancement they are usually more than willing to encourage you rather than feel slighted.
Scroll Down to Read Other Opinions
What's Your Opinion? Sign Up Now!I can't see how it's a turn on for her
In Short Term: -1 night of fun In Long Term: I will get more jacked. Plus I am dedicated to what I pursue and that's sexy
You see, there's a flaw with your logic. What you think and what she things ( or anyone who's turned down really will think ) are very different. You think " He's dedicated, that's hot! " and she thinks " This man just turned down my hot sex to hang with other dudes in a room that smells like balls. Am I that ugly? "
agreed with cromartie
Sexual Relationships 
Sexual Health 
Most Helpful Girls