“Projection.”
Most of it comes out that way, yes.
A lot of posters don’t seem to hold to the traditional understanding of premarital sex as immoral, but want to consider themselves champions of conservatism.
And therein lies your misunderstanding. Political conservatism, which is the focus of this site, is just that: political. Premarital sex as immoral (or sin) is a moral/religious issue, not a political one.
Do people with similar beliefs often overlap convictions on the subject? Yes. However, championing political conservatism does not, in an absolute matter, include making religious or moral judgements about others personal lives.