'Girls'? Pfff, how old do you think we are?
Personally I don't think I have any answer for you. I've never had a climax from penetration alone with a human partner. When mating with the dog, the stimulation can actually be almost too much and I actually don't feel at all inclined to touch my clitoris. Other times it is exactly what I need. I can climax from the penetration and knot vaginally, but some manual assistance is often required for anal.
I'm not all that hung up on an orgasm being the only outcome of sex however. Good when it happens but secondary to the overall experience.