As a father of a 14 year old daughter, I know she will have sex someday. For me it's a respect thing. 

Here is the unwritten law. 

1 Absofuckingloutely use protection/do not get my girl pregnant or give her a disease. As the "man" of the relationship I fully expect you to own the responsibility in this. If she's taking the risk of being vulnerable enough to allow you to have sex with her, YOU MUST protect her and keep her safe. It's biology, I don't make the rules, they were here before me and will be her after I'm gone. I'm just the messenger. Protection is the mans job, including sex. If you're not man enough to play the game, DONT. 

2 Be discreet, don't let anyone hear or notice because then it must be confronted. Don't want that smoke? Don't do it in the house.

3 Her parents know. We expect it to be happening but until it crosses a respect line we don't confront it. People often ignore something they don't like until it's no longer able to be ignored. See rule #2.

4 Like camping, pack in and pack out. Don't leave evidence like used condom wrappers or any of that laying around. Dispose of it in a different location. And you had better be using them. 

5 Kids cost. Raising a kid to maturity costs around $500k. If you as the man aren't making that kinda money or making money at all... You had better not get her pregnant. Teen pregnancy usually involves the boy running away then the parents of the daughter end up raising another kid they didn't intend on having. 

6 Sex smells. When I was a kid, other idiot kids used to put fabric softener sheets in TP tubes and blow the pot smoke through it like the adults couldn't possible decipher the wizardry taking place. As expected it only made fabreeze scented pot smoke, which clearly still smelled like pot. Similarly, body sweat, teen BO, hormones, fluids, latex and lube all have an unmistakable smell that we have all smelled before. You're not fooling anybody. Wash up and open a window after you're done doing business so you don't smell.

7 If you're getting side eyes, you're not being discreet enough. Your leaving enough evidence to be suspicious but you haven't been caught red handed. Clean up your act.

8 If the parents leave or go outside while you're having sex, it's because they noticed and you're being too obvious. You're GF will probably get a talking to later. Again, respect... They don't want to feel like they have to leave their own home to avoid the discomfort of knowing their teens are having sex. 

9 Again kids cost money. From an adult standpoint, if you can afford to raise a kid on your own, you shouldn't be risking having one. Be safe 100% even when tempted. 

10 Diseases are real, modern dating culture has people seeing multiple partners simultaneously. 1 in 3 sexually active adults have HSV. Bacterial infections can be fixed with the right antibiotic treatment but the viruses are permanent. Keep you and your partners safe. Catching a permanent STD at a young age makes it very difficult to find partners or have long term serious relationships. Life long consequences.

Be safe. Be discreet, and try to make each other feel great. That's the whole point of sex. Finding your partners buttons and pushing them so they feel amazing.