Not necessarily the only thing they're after, but certainly that there's an issue which could be that. Sex drives vary - some people, if they're not getting regular sex, it can start to really dominate their thoughts, some people (both men and women) simply need sex every day to be able to function without those thoughts dominating them, and if your own sex drive doesn't match theirs, then they either need to learn to do without, you need to make yourself available more often, or go your separate ways. It really can be a deal-breaker for some people, others can work things out. I'd just ask precisely what it is they want, how often they feel they *need* sex, and come to an agreement about when it is or isn't appropriate for them to hassle you about it. If you know what they expect from you physically, it's easier to figure out precisely where the line between them being reasonable and unreasonable lies.
Sex is, for better or worse, a fundamental part of a romantic relationship for most people, and yet something they mostly don't talk about. Just being open about your concerns can often fix that. Of course, they could just be trying to use you, too, but that's a judgement that I think you have to make based upon how they interact with you in situations where sex isn't, and cannot be, on the cards. Are they interested in you as a person and interested in spending time with you, in doing other things with you, when sex is definitively unavailable?
edit Whoops! I got to this post from the front page, didn't realise it was on the women's forum, I'm male and can delete this comment if y'all would prefer?
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"You're never more alone than when you're alone in a crowd"
-Captain Sheridan, Babylon 5
Music of the Moment: Radiohead - In Rainbows