Ease Me In (The more guarded approach):
[Leave some brochures about your method out in the house somewhere]. I left these brochures out because I want you to know I am using birth control. I love you and I want to share this part of my life with you, so I hope you'll at least take a look and let me explain why I made this choice.
Just Give It to Me (The no-nonsense tactic):
I don't like feeling like I'm sneaking around behind your back, so I'm just going to come out and say it: I'm on birth control. And it's not because I'm sleeping around. It's because I want to be safe and protected. Period.