You and your girlfriend make out, your naked bodies pressed against each other, crotches rubbing against each other. You feel a warmth in your chest. Your girlfriend pulls away and exclaims "That's more like it!"
Looking down you see your breasts were merely a delayed stage of your transformation. Curious, you cup your new chest. Your girlfriend gives you a moment to contemplate your changes then moves your hands out of the way. Her lips return to your body, but this time to your new breasts. She teases your nipples with her tongue and you really understand now why she likes it when you do that to her. You can feel your tail flapping with excitement.
"Seems like you're enjoying this!"
"Yes!" you exclaim and realize your voice has changed to match your new body. How did that tail change you so much?
"Would you like me to...show you some other benefits that come with your new form?" your girlfriend says, then bites her lip nervously.
"Have you always been bi?" you ask.
"No, I just have a mermaid fetish. I was going to wear the tail so you could fuck me as a mermaid, but I'm really liking you being the mermaid too."
"Eat your heart out."
"Oh, I was thinking somewhere a little lower" she says. Your girlfriend slides down your body, all the while rubbing her crotch against your tail, which she seems to quite enjoy. She's ready to pleasure you like you've literally never been pleasured before.