At Imbolc is the wedding between the God and Goddess, and of course it includes the consummation of that marriage. On Ostara their union results in conception (of the Son God, who will be born at Yule, after the Father God dies at Samhain...of course that Son will then become the Father at the following Imbolc when he marries the Goddess...).
So here is what I see as the differences between the three sexual celebrations:
- Imbolc is a celebration of marriage, of consummation, and of the sacred, vow-renewing, side of sex.
- Ostara is a celebration of procreation, of conception, of birth; another awesome side of sexuality, yet also not all it can be.
- And what is left for Beltane? Sex just for the fun of it! Because the pleasures of sexuality are also important.
What do you think?