She has to

Swallowing again Batgirl kneels down without thinking twice, wanting that closer look. Also without thinking she reaches out and grabs onto the dick that is already hard. After a moment she speaks up.

"Uhm. What. How does it feel."

"To be honest with you it feels kinda good, but also your gloves are a bit rough. Maybe take them off?"

"Sure thing." Taking her gloves off quickly she grabs hold again, it is so warm, and it looks tantalizing. Batgirl wonders how it tastes, then she wonders where that thought came from.

"So, notice anything that can tell you about its origin?" If Batgirl were to look up she would see a teasing smirk on Harley's face. But instead she is still staring at the cock, admiring it more than inspecting it, however the words do break through her haze and she responds without looking up.

"Uhh, not yet, looks good though. I mean, it matches your body well." Face now red with a mixture of lust and embarrassment Batgirl resumes looking at the dick, and the question once more enters her mind. It feels so good, how does it taste? This time she cannot resist licking it.

"Ooohh, that felt good Batgirl. You think that will help you figure things out?"

"No. I just need it." At this point all resistance is gone, the pheromone exposure has fully gotten to her and she is too horny to resist her impulses. And she has no regrets, it tastes amazing, forget licking she puts her whole mouth on it running her tongue around the dick in her mouth, trying to taste as much as possible. Harley moans and lets her explore without interfering further.

For now the only sounds are Harley's moans and the wet noises of Batgirl slurping on Harley's cock, and some muffled moans from the Bat. Reaching up with her hand she cups the balls, they feel good on her bare skin too, maybe they also taste good, swapping places her mouth moves to taste them while her hand continues to stroke the dick, she is long gone by now.

"I am about to cum Batgirl." This moves Batgirl into action, her mouth goes over the tip of the dick and her hand strokes it faster, wanting to taste what comes next. In a few more seconds Harley erupts into Batgirl's mouth and she swallows it all, the corrupting semen quickly taking effect. The start of the changes are mostly in the mind, Batgirl will not tell others about this, she will become addicted to it, and then there are the personal changes, she is going to become an admirer of asses, slowly but surely, for now she will just admire them, but eventually it will border on obsession. However, none of that matters to her yet, she still wants more, needs to be fucked now, so she strips off the bottom of her costume and turns around on her hands and knees shaking her ass enticingly.

"Please fuck me Harley, I need it." Harley just smirks again and moves behind her quickly thrusting in making Batgirl squeal with pleasure.

"Yes! Yes! Yeeeesss! Fuuuuck Meeeeee!" Batgirl drops down to her elbows and knees, the pleasure so overwhelming she cannot keep herself elevated. Face down ass up on the floor with her tongue out as she moans and squeals in pleasure, if she were in a more normal state of mind she would be humiliated, instead she is simply thoroughly enjoying getting fucked into the floor. In no time at all she has already cum multiple times before Harley finally cums inside her, this causes the physical changes, ass grows of course, so do her tits a bit. If anyone notices, well, she isn't the only one that has changed thanks to the water contamination.

Once more something that in a normal state of mind would not be good, Harley cumming inside her, is phenomenal in her pheromone and dick addled state of mind, it is so hot and feels so good it makes her cum yet again. This time she cums so hard she passes out already. Harley pulls out of her and waits for her to wake up, in the meantime she makes sure the windows are open and turns on the fan, this is enough for the first session, so she wants those pheromones gone.

After some time Batgirl wakes up, and with some embarrassment puts the bottom of her suit back on and quickly leaves with an awkward farewell.

"That was, uhm, great Harley. I will, uhh, let you know if, uhm, I figure out the source of that wonderf-. I mean source of your cock. I mean dick. I mean, uh, bye."

Harley is barely able to hold in her laughter until after Batgirl has left, but once she is gone she lets it out. This is going to be a lot of fun to make her go from this to her eager pet ready to help corrupt others. The slow game is so hot.