Yes, it's a religious rule. I'm not religious therefore I really don't care what these "rules" are. I'm aware they exist but I chose not to follow them. Personally I wouldn't want to have sex with someone until I knew they were the one. I wouldn't need the security of a marriage before having sex.
I believe committing adultery is having sex with someone else while you're legally married to someone. After divorce it's no longer adultery as you are no longer bound legally to that person. I know some people think sex with anyone else after you married once is adultery but I disagree.
Multiple partners aren't ideal to me. But shit happens; divorce, break ups, death, etc. so I believe if those circumstances arise and I move on and find someone else, I would not be committing adultery seeing that I'm no longer involved with the other person.
One thing I would not do is run around with multiple partners with people I wasn't sure was the one or cheat. As long as I'm with a person I will be committed to them and if something happens I will move on to someone else (which I hope wouldn't have to happen but with life anything can happen).
If we wait until marriage, great! If we don't, that's great, too! But we will decide what's best for us on what we think, not because its some ancient rule.