I have developed four stages of dating to protect me from pushing too far too quickly.
Stage 1: Making out.
When you first meet a girl and you are having a couple of date, just hanging out, that's called the "Making out stage". Do not call them your bf or gf. Just say "The (girl/boy) that I'm making out with currently.." Both of you are free to roam, technically. No attachments.
Stage 2: Hooking up.
You guys have gone on several dates and have made out or slept with each other. You are now the girl/guy that you are "currently hooking up with." Still no attachments, but you have a slightly diminished area of roaming, if you will. The guy/girl has shown signs of wanting to be a couple.
Stage 3: Dating.
At this stage is the time you need to have a quick talk and express your want to be a couple. It can be as simple as saying "I would like for you not to hook up with other people." I can't think of a simpler, more blunt way to say, "I want to date you and you alone." Now there is attachment and no chance to roam because NOW it's cheating.
Stage 4: Relationship.
After the 3 months "bliss period" where everything is perfect and you are getting to know the person, you enter the relationship stage where you or the other person need to decided if this is someone you want to pursue a full blown serious relationship. After the first three stages, this should be an easy answer. If you or the other person has doubts, walkaway. Be honest. It's not there. There are other fishes in the sea. Many, many other fishes. Trust me
I hope this helps. Good luck!