Trust in relationships
Honestly I don't think that this is rocket science. If you do not trust each other it is not going to work out. With this in mind, it is absolutely normal to think the worse (cheating of course) and that it is normal to have suspicious of the partner exploring beyond the current relationship. Personally, I learn a woman before I get too close. I'm relatively old fashioned about relationships and I don't prefer to do things like "fuk bitches get money" and "yolo" shit. I honestly get into a relationship because being around her makes me feel like a completely different person, not because I want sex.
Cheating
It's bound to happen, maybe not in all relationships, but there's usually a reason for it. It can happen when people enter a relationship to quickly or have no real regard for each others emotions. They are in the relationship only for sex and really don't LOVE each other. They are more just like two horny rabbits who were left along in a cage. They don't really care about each other, they pretend to. Obviously not all cheating occurs like this, in some cases, the cheating is something that occurs because of a fantasy. Perhaps they have been hurt and do not know what to do. But most people who cheat know exactly what they are getting into.
Rumors & friends of the opposite sex (ties in with trust)
If I get into a relationship, I already know that I trust this peson and that I love them for what I have come to know them for. Some people cannot say the same things about their partners. Having Rumours being spread around is EXTREMELY common amongst relationships, especially in places like high school. You know how I would deal with rumours Show her off. Hold her hand in public and kiss her in front of people I know. People will stop spreading rumours if you don't care about them. If you are REALLY suspicious you can confront her about them.
Getting back together
It's possible, but really only will happen in serious, or in shallow relationships. If they are truly committed and they realize they messed up, sure it can be possible. The same thing goes for relationships that are prone to fail from the start, they get back together for sex.
Staying friends after etc....
Honestly I think staying friends is going to be really awkward especially if things got serious.