It should have hurt. The crunching and reshaping of bone. Muscles tearing and fusing together. Your very DNA being rewritten to include the new fishy elements that were being woven into your core. Your feet had begun to flatten and toes became webbed and then not toes at all, as they were forming into fish fins very similar to those of the mermaid under you right there. You could feel your legs actually being forced and pushed together, as they fused into one limb instead of two.
And yet... it didn't hurt even a bit.
Of course, your senses were being kept very occupied while pumping deep into the mermaid that was keeping your body locked inside of hers. Or at least, keeping your cock locked inside that very warm, wet, fishy tightness. The pleasure that was being brought about by this very act was overriding even the small twinge of pain that your transformation was bringing about. As your merged legs grew longer and longer, forming into a teal-scaled tail and fins, you were happily oblivious!
Her hands reached up and gripped yours and placed them on those very curvy, heaving tits. You could've sworn they had swollen and got even bigger as you continuously pumped deeper into her, and then faster into her as well. She moaned at you, her voice crying out "Cum inside me! Please...please fill me, my merman!!!!" Obeying her, you pumped harder and faster, her speed matching yours. You felt something grip your new tail, as the mermaid's tail coiled around yours tightly. Something about that did it, and moaning loudly, you complied with her request, filling that mermaid womb with every ounce of your new merman seed. Her back arched up, breasts heaving upwards into your chest, her moans of pleasure filing the air loudly with yours!