Honestly, I can't think of a sexy Trek episode. Not one. Especially not the ones that are *supposed* to be sexy.
Star Trek has the sexuality of an awkward, insecure teenager. It's obsessed with people in tight clothing, women in mini skirts, women in catsuits, men who lose their shirts at any opportunity, exotic scantily-clad alien females, bad boys and bad girls dressed in black latex and rubber, and biker dudes and biker chicks with huge cleavages. There are a lot of attractive images of people, a lot of desire and hinted sexual tension. But actual sex scares the hell out of it (not to mention relationships - that's even scarier! aaaargh
), and when it does happen, which is not that often, it is clumsy and unsatisfying, while any 'relationships' are usually very short and awkward. (Though there are a few exceptions to the latter.)
There are quite a few sexy Trek characters. But sexy episodes? Not really. Trek scenes showing people having sex tend to fall into one of those two categories:
1) make-out scenes leading up to sex: usually involve a most generic, dullest dialogue and setting possible. Holodeck is often used to create an illusion of a stereotypical 'romantic dinner' with some jazz/evergreen music playing in the background. It involves people speaking some generic flirty lines and compliments and finally kissing, at which point background music starts playing; scene ends.
2) post-coital scenes: usually involves two people lying in a bed covered with sheets (it's good to see that they're so determined to hide their naughty bits from someone they've just had sex with) and having a conversation about the fact that they've just had sex.
But it could be worse. It could be like NYPD Blue, with its obligatory generic sex scenes at the end of every episode which proved that, believe it or not, you can be bored to death by the sight of naked people rolling around in bed.
The sexiest episode I can think of is The Enterprise Incident with the scene between Spock and the Romulan Commander. How funny it is that the sexiest Trek scene I can think of involves two fully clothed people touching each other's faces.
