Lets get to the other point. You got this friends with benefits over here. Than Sally pops up and she's got a job and everything. Want a serious relationship. You start dating her while your still doing Betty on the side.

And you think your going to get away with it.

Right. Well, you meet someone else that's got a job and's looking for a relationship. You've been banging this other broad but you've not actually got a girlfriend and you are available.

You are actually interested in a serious relationship and so is this other lady. You are both looking for the same thing and you aren't playing her. But maybe you did just meet her and aren't sure if she's girlfriend material yet. You've got two women interested in you and no future with one of them and possibly a future with the other.

It often happens that people "cheat" at the beginning of relationships but it's debatable that it's cheating because you're just getting to know each other and didn't necesssarily say that you wanted to be their one and only yet. If they've got principles about no sex before monogamy I guess that you have to agree to that and you can't shag anyone else while you're dating her but you maybe have your old FWB as a fallback if it doesn't work out.