Does anyone else feel uncomfortable closing on a girl who's a friend?

I have a really hard time closing with girls that have crossed that threshold from being random chics that I've met, and are on their way to being friends.

It's come to the point where I don't bone these girls I've come to know. It just feels way too awkward, like there's too much on the line. Because I don't want to jeopardize anything (as cliche as that sounds).

Like... a co-worker of mine who I've come to know this past week... I ended up back at her place, in bed, and I really didn't want to throw down because I didn't want to jeopardize anything. Of course, I totally did, but now I regret it.

Anyone else have this problem?