[So he truly doesn't understand. No wonder he's managed to find himself running into problems with humans and sex.]

When it's with someone you have any sort of real feeling for, it's intimate. It's being vulnerable with someone else, especially if you're being penetrated. It's built up as the highest romantic act there is, for anyone who believes in that. It doesn't matter if it involves pain, pleasure, or both. It's closeness and trust. A step towards something more.

And if you want the scientific spin on things, in a human brain it releases chemicals that produce feelings of bonding and affection.

[He gestures.]

As I mentioned, that's most people. Some people have personalities, maladaptive traits, or enough experience in a way that allows them hold onto more distance.

But almost always, every part of it is messy and irrational.