Trans Venus

Trans Venus is a manga by Tamaki Hisao; it tells the story of a boy named Suduo Takehiro who one day walks into some strange woods while traversing a mountain trail and is saved from a gigantic centipede monster by a nymphomaniac goddess who can not only take over his body (turning it into hers in the process) but can also transform him into a girl.

However, while Takehiro is as hormonal as the next guy, he quickly begins to despise his savior Selda, not only because they now have to share a body and she likes to take over, but also she likes to mess with him for shits and giggles, not to mention her own raging sex drive quickly becomes his personal Squick.

Unfortunately, her power is kinda needed, as she's the only person who, through Takehiro's body, that can stop an Alien Invasion.

Possibly one of the weirdest and more insane takes on the Gender Bending genre you'll come across. It was released in two volumes, both of which have gotten a Fan Translation.

Tropes used in Trans Venus include:

Alien Invasion: The monsters that attack Earth are basically going to do this en masse, as Takehiro soon learns not long into Volume 1.
Body Surf: Selda's specialty, but once she picks a human host, it takes her awhile before she can pick a new one.
Cosplay Otaku Girl: Invoked and lampshaded, as Selda turns Takehiro into one, and people who notice his outfit think he (or rather she is from a Magical Girl anime convention.
Ecchi: It's chock full of sexual content, but the naughty bits are generally hidden most of the time, making it rather softcore at worst.
Fetish Retardant: In-universe, Selda becomes this for Takehiro, because even though she's basically a Hot Goddess skin riding his body who likes to wear Dominatrix style clothes (when she wears them at all) and is more than happy to do This and That with him and help him get laid, her constant getting on his nerves makes it hard to appreciate any of this.
Gender Bender: Selda has to turn Takehiro into a girl because that's the only way she can manifest her full powers, at least that's her excuse for when he actually needs her help. The other half of the time she just likes to drive him up the wall.
Sexy Mentor: Selda, not that Takehiro can appreciate it too much given how much she likes to annoy him.