So this theory explains why the snails have sex, but it doesn't so clearly explain why the worms have sex -- since they are the parasites in question.
I don't know why there has to be just one reason that organisms have sex. If there's anything we know about biological systems, it's that most structures serve multiple functions.
(via sex). This is why.
I'm inclined to agree. I'm not sure why the author limits themselves to parasites. Parasites are not the only selective pressure that can be addressed via genetic recombination.
Parasites are distinct from most other selective pressures: they constantly adjust to whatever adaptations the host has come up with. Other pressures do not have this property - the desert doesn't get much drier because some plants adapted to the lack of water.
This also explains why worms have sex - even if they don't have parasites of their own (they most probably do), they need to adapt to their hosts' countermeasures.