Can pre-cum happen before sex

Yes.

Pre-cum is a male's natural lubrication. When a male is erect, he will normally excrete varying amounts of pre-cum with or without penetration.

And, though it's not part of your question, remember that pre-cum often contains sperm! So, if you're going to be using a condom or other type of barrier contraceptive, be sure that it's properly in place as soon as the male is erect.