I'm not a TSA policymaker, so I couldn't say for certain, but I'd guess it's a combination of [a] tradition and [b] lawsuit avoidance, particularly from women.
Tradition is fairly self-explanatory - once upon a time, everyone was assumed to be straight (and indeed, most people still are), so the "inadvertent sexual thrill" thing seemed a fair justification. Even though things are more complicated now, it's still policy.
The second bit is more likely linked to how women and men are perceived socially. Sexual predators are almost always male, and while that doesn't justify treating every man like he's a potential predator, women who are getting felt up by a male agent are far more likely to complain/sue than women who are getting felt up by a female agent (or men who are getting felt up by a male agent, as there's the whole "men can't be raped/harassed" attitude in our culture as well).
Frankly, I think the distinction is kind of ridiculous and arbitrary, but it's not too hard to find evidence that I'm in the minority. Hell, for a much lower-stakes example - about a decade ago (I think), Ann Landers answered a letter from a woman who had encountered someone she was certain was a man in drag in the ladies' restroom, and was wondering what she should say in the future to make it clear this wasn't acceptable. Ann (who, bless her, was remarkably progressive for her generation) pointed out that the person might have been a woman who looked masculine, or a transwoman, and that people should be able to use whichever restroom made them most comfortable. You should have seen the firestorm of hate mail this unleashed, with women claiming that she was giving any pervert or predator free license to use the wrong-gender restroom. (Like there aren't locking stalls in most public restrooms? Give me a break.)
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Tradition is fairly self-explanatory - once upon a time, everyone was assumed to be straight (and indeed, most people still are), so the "inadvertent sexual thrill" thing seemed a fair justification. Even though things are more complicated now, it's still policy.
The second bit is more likely linked to how women and men are perceived socially. Sexual predators are almost always male, and while that doesn't justify treating every man like he's a potential predator, women who are getting felt up by a male agent are far more likely to complain/sue than women who are getting felt up by a female agent (or men who are getting felt up by a male agent, as there's the whole "men can't be raped/harassed" attitude in our culture as well).
Frankly, I think the distinction is kind of ridiculous and arbitrary, but it's not too hard to find evidence that I'm in the minority. Hell, for a much lower-stakes example - about a decade ago (I think), Ann Landers answered a letter from a woman who had encountered someone she was certain was a man in drag in the ladies' restroom, and was wondering what she should say in the future to make it clear this wasn't acceptable. Ann (who, bless her, was remarkably progressive for her generation) pointed out that the person might have been a woman who looked masculine, or a transwoman, and that people should be able to use whichever restroom made them most comfortable. You should have seen the firestorm of hate mail this unleashed, with women claiming that she was giving any pervert or predator free license to use the wrong-gender restroom. (Like there aren't locking stalls in most public restrooms? Give me a break.)