Legislation, societal pressures, and familial scorn shouldn't be reasons behind it, but I do think it is a good idea.
Maybe it's because I'm not a base hedonist, but chastity does display a great deal of discipline, and is very telling about the kind of personality the person has, regardless of their religious or philosophical leanings.
EDIT:
SirBryghtside said:
The only point of sex is to have children
If you had any grasp of the real world, you'd know that's an outright lie. Sex is partly for the purposes of having children, but also for the purposes of pleasure/connection between two people. I could easily say some rhetoric here about how animals have sex for pleasure as well, but that's part of the reason I didn't want to get into this argument.
No, sex
is for the purpose of having children, it just feels good as an evolutionary measure to ensure creatures do it. It's the same reason creatures from reptiles-up fertilize internally rather than externally.