Adding a sexual component to your BFship is risky business, whether you are adding it as a component of morphing your friendship into love, or whether it’s just for… convenience. You would need to be very, very sure that s/he was on the same page as you are, even on the subconscious level. I’m not just being a prudish wall when I say I would advise looking for sex within a committed relationship, and not potentially ruining something as sweet as a best friend relationship for brief pleasure.