Yep, a guy.
In case anyone wonders, it was in either chapter three or four that Amanda has a brain tumor and dies, that's why she's only in the flashbacks. As for Nick sleeping with everyone, as the story progresses you see how he evolves from just being with Amanda, then Amanda and Anna, to just about everyone. As for being married and having sex with multiple partners, some people see sex as an expression of love and caring, like a really good hug. There's a difference between those you love and those you are in love with.
Just my opinion...I'm sure it's not for everyone.