Both of mine are fixed and they both still do it. Riley was fixed at 6-months-old, and I got Sadie fixed about a month or so after I got her (she went into heat the day after I found her, so I waited through that) at a year old.
Mine do it as a dominance thing, and they only do it to each other. Sadie will do it if Riley is sitting for attention from someone, and she sees the opportunity (she likes to try when he's sitting, I guess since he's so big). Riley will occasionally do it to her when they're excitable and she crosses some line or another. Sadie doesn't completely accept Riley as dominant, I think. She knows it, but she'll occasionally decide to try to hump him just to let him know that she's on his heels.