Do you mean they have to show more sex in order to make relationships appear more real? If so, I don't agree. They tried that in ENT and it didn't work. They showed some sex, but it was anything but romantic or mature. In my opinion anyway.I also think romance, as opposed to friendship, requires a certain mature honesty about physical relationships, and Star Trek has usually aimed too low for this (in television, if not books).
It's not only Trek that has trouble mixing sex and romance in a mature way. I live in a country that has more explicit sex on prime time tv and in movies than violence, but the sex usually doesn't involve any real romance.
I agree that it's not about showing more sex. It's understanding how to portray real physical relationships in a mature way. The flipside of twee, melodramatic romance is pure physicality, devoid of tenderness.
As I wrote earlier, part of the problem is lack of continuity. Many relationships are too short to be developed in any depth.
But it's also that there's a divide at work: love without the suggestion of carnality, and sex without love. Both are products of a conservative approach, which has trouble dealing with sex in a grown up way.