Sex isn't thaaaat better than typical masturbation. It's not like your orgams is going to change or anything. It's more the intimacy of it that's actually stimulating.
To put it in other terms; It's like when someone puts their hand on your shoulder. Most people know the sensation and it doesn't change a whole bunch regardless of whether you or someone else does it.
Most of the time when people you are indifferent to/aren't attracted to do it, it's not going to be particularly fun or memorable for either. If someone you have genuine chemistry with, and done in the right place at the right time, it almost feels electric.
If you cast your memory back to times in your life when people have put their hand on your shoulder. It's more the context you remember, along with your feelings, rather than the action itself so much (unless you're trying to get off. Then idk)
That's basically my interpretation of sex anyways