While some parasites clearly manipulate the behaviour of their host to enhance their ability to spread to new hosts (think of alteration of limbic function by rabies leading to biting behaviour or reduced fear from predators in many instances), do they ever alter sexual behaviour?
There is evidence that many organisms discriminate against mates that are parasitized. It has even been proposed that positive selection for bright colours, large appendages, elaborate feathers/fur/etc is a surrogate for selecting mates who can support such useless extravagances... and are therefore free of parasites.
Rats have been shown to be able to identify parasitized individuals by smelling their urine, and avoid mating with them as a result. Female rats appear to prefer the scent of males that are free of lice over lice-infested males.
So do parasites ever lead to increased mating success? Snails (Biomphalaria glabrata) exposed to to Schistosoma mansoni respond by increasing their reproductive output. This is an adaptive response that enhances reproduction before the parasite causes host castration. Makes you wonder how they know?!
Milk-weed leaf beetles (Labidomera clivicollis) also increase their reproductive effort in response to parasitism by a sexually transmitted mite Chrysomelobia labidomera. This is where things get interesting. When a parasite is sexually transmitted, making it's host more promiscuous and/or more sexually attractive would be a great strategy.
Just imagine how rabies might effect its host if it were sexually transmitted...