Well I'm in a serious relationship now, but it's a good question which my girlfriend and I have differing opinions on.
She would say that dating someone just for sex is wrong. I know because we've talked about this hypothetically (not that I'M doing that, we were talking about another friend).
My feeling on the matter mirror's keyser's:
As long as both parties have this understanding, I have no problem with it.
Single people want sex too. Nothing wrong with that in my mind. But I would say that both parties need to be on the "same page" and have the same understanding about what the activity and contact does and does not mean for them.
I might add that being in a "dating" relationship and having sex for that long (you said over a year, Min), I can't help thinking that you WOULD develop feelings for the other person in that amount of time. Not saying you'd be ready to tie the knot, but that you would at least have SOME feelings. And I think that's where Terry's comment applies:
Using another person (by consent or otherwise) is a haunting and degrading awareness implying something deeply deficient inside you.
If a person has been seeing someone regularly and having sex with them for over a year, and doesn't care one way or the other about that person (that is, it's JUST sex) I agree that it might be an indicator of an emotional problem. Either this individual has an inability to form personal attachments at all, or this is not a person he/she is at all compatible with. Either way, I tend to think that it would be a tight rope to walk. This girl/woman may not have any feelings for the guy, but what if this fellow formed an attachment for her?
Like so many other relationship related questions I think it comes down to openness and honesty.
I think it would be cruel to string someone along who has other intentions or feelings that are not shared just to satisfy your own personal needs.
Of course, if full disclosure is made, there are open lines of communication and both parties know EXACTLY where the other stands on the issue, then BOTH have made that choice for themselves. I'd say in that case, anyone has the right to decide their own love life, even if their decision ends up causing them pain down the road. Is it the wisest decision? Maybe not. But it IS one they have the right to make.