But don't be.

First of all - DON'T start thinking about things to kill the reaction - that will almost certainly also kill the mood and ruin the kiss.

It's not offensive and is, in fact, rather supposed to work that way. If you're worried about it bothering her, though, particularly if she's a bit scared by the concept of going that far, then turn your body slightly when you kiss to keep that area a bit less in contact. Make the primary below the belt body contact the edge of your hip.

If she should move so that you're more in contact when you do that - be aware that might be a signal not to try and hide your reaction. Keep in mind that it's a compliment to her that you're that attracted. She may appreciate that and want to enjoy that attraction. Be advised that could lead to things proceding a bit further though!