Quoted from jonnyo:

She doesn't want to feel like she's number 2 behind your hobby. Even if she is, just fake it a little.
Either that or she's needy. No way to really know without actually knowing her.
Having a girl jealous is not a bad thing. Here's how you play it: First you describe a sex fantasy to her that you want to try. Naturally she says "Ew gross!!" Then you go back to working on pins for a few days. She says "Pay attention to me!" This next part is key. You then get quiet and a little teary-eyed, and look off into the distance, and you say, "I know this is hard for you... it's just that when I was a boy my father and I used to build things in the garage, and this is how I remember him." Then she'll feel guilty. Start the make-out session and then when things are getting hot and heavy you go "hey remember that little idea I had with the rodeo clown and the trampoline...?"