Honestly, it's way more soothing than it has any right to be. But he was holding on to his erection for such a fruitless amount of time that the stimulation brings him relief more than excitement, and it's way too fun to have her dick pushing in and just barely reaching his internal testicles. He'd be in trouble if she could get to his prostate, but like this? He can just relax while he drills in deeper, stretching her further and further and feeling her flesh stretch eagerly to accommodate him...like this is always where it was meant to be.

Far enough inside of her, the bulk of his tentacle actually starts to show as a lump on her lower stomach, where the squirming movement it does to pull itself in deeper can be observed.