Just because they're not sleeping with their colleges it doesn't mean they're prudes. Anyway, there are some trekk babies. There are some small romances and there are some "they just did it" scenes. In one of the TOS episodes Kirk even seemed to imply Spock was sleeping with a woman (this side of paradise). It's not that they're all virgins. It's just that they don't sleep with their closest colleges and best friends. Which is great! Because that means there is less time wasted on boring soap opera stuff. The love stuff going on between Riker and Troi, Picard and Crusher, Nurse Chapel and Spock. It was subtle and never compromised the plot.
I'll admit I am a big fan of "innocent" shows like the A-Team. I like how "clean" they are. They are pure entertainment. I also like the less innocent shows like Dexter or Hung. But it's nice to actually have one show that doesn't focus on death, sex and love.