I guess that initial spark that you get when you meet someone new can sometimes be so powerful. With all the benefits that you get from LTR, that spark is something that can 'force' people to infidelity. As for the 'empty feeling' you get after hookups, I kind of get what you mean but it's maybe more common with women, at least I would think so..
But I would assume my opinions on this would probably change as I get older and more mature.
For the OP - I would just be direct about it on the 1st date, I usually do it over messages