Here’s the plot that I can remember of it:
A guy is wandering through a wooded area when he suddenly hears what sounds like an orgy. As he investigates, he gets captured by some sex monster. He is then saved by a goddess or space warrior that takes control of his body and transforms him into her own image to fight off the sex monster. Now, she is bonded with him and he can be transformed into a girl to fight off other sex monsters invading Earth. By the end of the doujin with all of the monsters defeated, he decides to remain a girl and becomes lovers with his new girlfriend (that the leader of the sex monsters has turned into a futanari and the Goddess didn’t undo that transform since the MC seemed rather happy with it)