Every act of sex is meant to be an act of love, and enough to encompass new life should that occur. There are natural means to prevent conception.
Here's the problem. Where do you differentiate between "natural" and "artificial", and why is the married couple not desiring more children approved in one case while opposed in the other? If you take the approach that every sexual encounter MUST be done so children can be produced, is it not then the DUTY of the married couple to, for example, have sex during every one of her peak fertile times?
How much do you value sex?
I value it quite highly because it creates a bond between my God-given spouse and me that is unique to us and no one else.
Do you value it enough to be open to its full purpose?
We have two children and did not desire anymore, so yes, you could say I was open to one of its purposes. We continue, however, to enjoy our sex life long after it can possibly produce more children. If we are to limit sex to only procreation, no one would be having it after menopause. Therefore, it has multiple purposes far beyond merely producing children.
Do you value your partner that much, and does your partner value you that much?
Yes, we value each other quite highly.
Do you value eating? Is bulimia a sign of valuing food or is it a sign of preventing food to do the work for which it is designed?
Now that is intriguing. It would sound like you're arguing that someone who deliberately chooses to limit their caloric intake because they desire to lose weight is not valuing food for it intended purpose. You would not, for example, argue that obesity is a sign that someone is using food correctly. I would argue that a person who is clearly at a healthy weight IS using food correctly for its intended purpose. Here's where it's different. Enjoying sex with your spouse beyond having children is not harmful to you at all. Eating more than is healthy IS harmful.
God designed sex to have multiple purposes. When you read the Song of Solomon, for example, you do not find the writer thinking about having children or getting his wife pregnant. He desires her and she desires him apart from children. The "full purpose" of sex, therefore, does not necessarily involve children. I would expect, for example, that barren couples should have a full and exciting sex life, praising God for their state precisely because He has freed them from responsibility for children for other purposes.