As man, I prefer to cum inside my partner and I have many reasons for this.
First of all it gave me a sense of domination, like I've just marked the girl I've cummed in. Secondly I really like to stay motionless a little bit out of her and then when the ejaculation starts feel the pulsations and make them feel to her, when the sperm pass across my dick and liberates inside her I can feel pure pleasure, also make her feel my penis pulsate gave me a sense of strenght cause I'm pumping my semen inside her using only my coccygeus muscle.
I also really like to think about the sperm not as material destinated to procreation, but only like a means to reach a greater pleasure, for me, like I said above, and for my partner, giving her a warm and cozy liquid, and a pleasurable way to ratify my possession on her with an invasive method.. I can represent this like a sort of spider web, it's sticky, you can't easily get rid, but is so confortable that you want to fall into it.
Once I had sex with one friends of mine, she was on the pill but she didn't like the cum inside her. So when we went to her contryside house to have sex I've passed a week without masturbate nor ejaculate, then we went in a room with a mirror cabinet. After all, at last I sat down on the side near the mirror and I asked her to put herself in the reverse cowgirl position, so we restarted to fuck, when I was near the ejaculation I grabbed her by the chest with my arms, I pushed us back a little to expose our genitals fucking to the mirror and then I said to her " look at my dick in the mirror now" and I started to penetrate faster till I reached the orgasm and I started to ejaculate, she did not even try to stop me, she resigned to see my dick pumping my semen inside her, I remember it was a lot, that time I jizzed so hard I was thinking to shot my sperm into her stomach. In this case I cosider really satysfing to come inside when she won't, cause it states my possession on her and I can also convert her to the pleasure of the coming inside.
(If you want to know, after I cummed inside her we made sex a second and a third time, and I made creampie again without asking her permission whereas the third time I cummed in her mouth as she wished, what a day)
Actually with my new girlfriend we start to have great sex and I cum inside her vagina anytime, but I also really like to cum inside her mouth and watching her swallow for the same reasons of the creampie more or less.
I forgot to say that I really like to cum inside also because I love to have an orgasms in a warm and wet place.