He wanted Rachel to be his wife, his lover, the centre of his whole world; a girlfriend implied that he would see her from time to time, that she would have some kind of independent existence away from him and he didn't want that at all.
. . . .
(Marcus:) "How do you know you want her to be your girlfriend?"
"I don't know. I just feel it in my guts." That was exactly where he felt it. He wasn't feeling it in his heart, or his head, or even his groin; it was in his guts. . . .
The whole chapter is about love, and how one knows, and what it feels like. Much of the rest of the book is, too.