Given the covenantal nature of the church (the church as the bride of Christ), sex and marriage, I think, can be seen similarly to the Eucharist and baptism. In baptism we are grafted into Christ, in the Eucharist we partake the body of Christ. So, you could argue that marriage is a sacrament derivatively, because it is a sign of a sign.
And now it makes sense why you need to be baptized first before you could participate in the Eucharist.
(I might be wrong, but at least I can’t be accused of classical liberalism, as this idea was not born out of personal experience.)