My partner believed I orgasmed the first few times we had sex. Why? Because I was literally screaming in pleasure. Was I faking it? No. But that's what he initially thought when I told him I hadn't come, that I had been faking it.
I think part of it is that penetrative sex is a very different pleasure than clitoral stimulation. Whenever I orgasm via clitoral stimulation it's a sharp spike of pleasure that washes through my clitoris and general area, and it's usually fairly silent. I tense up and then relax.
But when I am having penetrative sex, the pleasure is full bodied, centering in my core. Every thrust shifts my uterus, kind of like the feeling I get when I'm swinging on a swing or going on a rollercoaster, but more pleasurable. Waves of pleasure roll through my lower belly. Add to it the other tactile pleasures - kisses on my neck and ears, my hair being pulled, ASMR from the sound of his groaning...
I would choose penetrative sex with my partner, without orgasm, over masturbating to orgasm, almost every time. It's more pleasurable.
It just doesn't (so far) follow the process that I have learned to associate with orgasm. The build of tension and release.