[DrPhil]

Sometimes things are about how we present it rather than the overall point.

For instance, if someone thinks you can afford to pay but want the other person to pay half simply as a matter of principle, it can seem silly and like you're trying too hard to prove a point. If it isn't like you all are negotiating the things that he pays for in your relationship, but are only discussing birth control costs, he may wonder "why is THIS the thing that you want to negotiate?"

That can spark annoyance and he can feel like you need to just continue paying as you've always done and find something else to complain about (depending on the tone of your conversations). Afterall, responding to his resistance by saying you'll stop taking it and therefore sex will stop (I assume), is a threat and ultimatum that you have yet to act on. Threats and ultimatums, especially those that you have yet to act on, tend not to work well in relationships. They work better on children.

(Some couples share these kinds of expensives and some couples don't. There's no right or wrong, it's just a matter of what the couple negotiates.)