I've reached orgasm from penetration alone only once when I don't know what happened, but whoaaaaaaa did IT happen. Kind of sucked though because he pulled out right as it happened. Sigh, it was forever ago.
Other than that one time it had to be when the man was at an angle where he brushed my clit with every in press, or if I rubbed my clit against him...
Those angles are hard to achieve, one I need my knees open and it's inevitable that I'm asked to wrap my legs around the guy, two the guy likes to push up on his arms so he can watch which means I'm less likely to get any clit stimulation because he wouldn't press deep enough that way to brush against me. It's never happened from behind or when I was on top.
The few times I thought I was going to orgasm from penetration alone I would say something about how much I liked it, or make a noise that always seemed to lead to the guy changing whatever motion he was using that felt so good. Which means I lost the feeling and had to start over again.