Being uninterested in sex doesn't mean one would be uninterested in reproduction. Especially if your species is built without the possibility of sexual reproduction to begin with, but is capable of asexual reproduction, or reproduction by proxy.
And the symbionts might want to play sex with their hosts just for the curiosity value. It must get pretty boring in there if one doesn't experiment every once in a while. Why, some of the older slugs might be suicide junkies, especially savoring the moment when the host screams and dies on them...
Timo Saloniemi