I offer this Key with the disclaimer that you NEVER owe someone sex. It doesn't matter where they've taken you, how much money they've spent on you, or even what you've promised. You are allowed to change your mind, and you're allowed to refuse to do ANYTHING you're not comfortable doing. In fact, it's your responsibility to do so.

Assuming, however, that you're both consenting adults, remember that reciprocation is an important part of balancing a sexual relationship. No one needs to keep score, but things shouldn't get too uneven. There's nothing wrong with working out a trade (I know a woman who enjoyed trading oral sex for back massages- simultaneously), but you can be more subtle than that too. If a partner has just spent a long time focusing on your pleasure, it might be appropriate to return the favor. Also, if you've put a lot of effort into pleasing a partner and reciprocation is not forthcoming, feel free to ask for whatever you want. An obliging sexual partner should be thrilled to know exactly what you want, and should be (assuming you haven't exhausted them completely) very willing to indulge you.