Surprise her and ravage her backdoor

You sneak up some more until you're practically over her, yet she still hasn't noticed you, at least not until it's too late. She notices your shadow and tries to flip herself to her back, but she's not quick enough, and your body is pinning hers against the grass, the bulg in your pants rubbing between her surprisingly cushiony butt. She struggles in vain, making a series of noises that just sound like 'glub-glub' to you, but all this movement just makes you more excited and eager to fuck her. You swiftly shuffle you trousers and pants off, reavealing to the air your stiff cock. You waste no time spreading her ample asscheeks apart and finding her slick oily pucker; viscous, transparent slime with a green tint oozes from the anus, clinging to her unkept asshairs, and dripping onto the grass in dollops. At least entering won't be a problem.

You push your tip against the anus, easily working your way into this nice, tight hole. It's not warm, as expected from something that looks like a fish and probably cold blooded, but the friction will heat things up. Realising what you're doing, the woman stops and lays absolutely still, letting you do whatever you want. It'd look strange to an outside observer; a man fucking a scaled, webbed woman who's only reaction to it is to blankly stare and blink and occasionally 'glub' like a defeated animal. It kind of takes a little bit of fun away from it, but it's not like you're going to stop; this ass feels so good! She 'glubs' as your cock hits her sphincter, and she clenches her buttocks tightly. It takes you by surprise, almost causing you to cum right then, but you compose yourself. Your arms wrap to the front of her body to fondle at her breasts. So soft! You expected them to be scaled and rigid, but they're surprisingly delicate and supple, like soft bread dough.

She just continues to lie there, breathing softly.

This annoys you a little; here you are, fucking her ass wildly, fondling her breasts that would make a normal woman moan in exctasy, and all reacts with is an occasional 'glub'. Maybe if you cum she'll react somehow. You stop holding back and release your orgasm of seed; your hips bouncing against her buttocks like a trampoline, your head tilted back, a series of grunts and moans escaping from your lips...

...All she does reacts with is a 'glub-glub'. Come on, do something more than that! You egress your cock, spraying a wayward spurt of cum covering her glimmering, scaled backside before flipping her over. Two wide ochre eyes stared back at you, her face neutral. Wait... Did you forget that she was infected? Of course she was, how could she not be, but in the hurry to get to her and surprise her back door, that completely slipped out of your mind. Too late do you realise that your hands and feet are starting to break out in scales, your fingers forming a thin, translucent web between them. Not that you mind anymore mind you, the infection already at the point of changing your mind into a more beastial, primal state. Now your instincts tell you why she was so unresponsive during sex.

Fish fuck in the water. This perticular variety of infected was just out of her element, that's all.

You both dip into the water to continue fucking, your hand scratching at your new set of gills on your neck.