As someone who consider himself pagan (with qualifiers), i thought i'd chime in though i don't know how helpful i'll actually be.
Pagans often place great emphasis on the idea of immanence. If you're not familiar with the term, it means the belief that the spiritual isn't "out there" but right here, and can be experienced through things other religions might consider mundane or 'worldly', such as sex.
Sex is powerful and primal, and so too is religion/spirituality. Combining the two can produce an extremely heady and even mystical experience.
Think about how closely connected you can feel to a sexual partner or lover. Introduce spirituality, and you have a way to connect with your god or gods very powerfully as well.
You mention that your wife said she had a "profound" experience during her first and only 'great rite'. It seems, to me at least, that it's important for her to have a pagan lover because she wants someone who she feels can help her recreate that experience.
Sexual rituals are actually something i've discussed trying with my own non pagan husband, because i'm a worshiper of Dionysus, a god with sexual aspects. I think it would be an appropriate way to honour him, but it goes beyond that. Like many people who worship a deity, I want to connect with my god in the most intimate way possible, to feel his presence as closely and powerfully as i can, and sex has the potential to give me that to a degree that prayer alone cannot.
I hope this has helped in some way,