I think I'm a little the same way as you are, HD. I normally need to be very much in love already to feel physical attraction to someone, and that's why I'm not really great at casual sex. Some people I am good friends with say that the great thing about one-night-stands is that you can totally let go and never have to worry about seeing the person again, but I find myself completely shutting up if I can't trust the person as a friend or as a lover, which for me takes a longer time than one night or even two to develop. For the sake of new experiences, I've had sex with people I'm not head-over-heels with, and although the desire was there, it took a longer time to summon (a lot of kissing and cuddling, to very little effect at first).
I would normally always recommend being upfront. I have a friend who is somewhat nervous about getting close to new people because of her inability/disinterest to engage in penetrative sex, and I think that is something that is good to bring up early on when the discussion turns into sexual histories and possible futures etc. If you are anything like me, you might talk about having sex for a while before actually doing it. Something as simple as 'I'm open to meeting new people, but not really in a place where I would be hugely interested in sex right now' might do the trick, or at least open up the conversation. If you are far enough to call somebody your new bf, then there should be opportunities to bring this thing up naturally.
Me: bi female in my twenties