Funny

If you didn't enter a relationship with them, then it is not technically cheating. However, if you really like them to a point where you'd like to be in a relationship with them, then the decent & morally right thing would still be to wait with dating other people. This is similar to people who just broke up. You're expected to still have emotional feelings to your former partner and while you're technically single now it would still be very weird if you'd just hop onto the next person because it questions the honesty behind your feelings for said person. It's simply decency, not just towards others, but also towards yourself. Either way, in your example I would at the very least communicate whether they'd be okay with you dating other people if you're at a point where there's clearly something going on between the two of you.