Reason with a teenager? Thats new. Show them no weakness or yer life will become a living hell...if ye think its been hard work up until now, then sorry, that was just the warmup. Its about to get so much worse
I have a 15 year old daughter...advice? Dont teach them how to drive when they 11 and then expect them not to steal the car keys...fit an alarm. When it comes to talking about the birds and the bees, yer kid laughing at you means they could probably educate you...ye can live with the lie that they just embrassed if that helps ye sleep at night.
Be a responsible father and tell them truth about how terrible yer life has been since they were concieved. Yer social life down the toilet, no freedom, no money...all the most fun parts of becoming a father basically. Buy them some rubbers but do it discreetly and whatever ye do, dont tell yer missus if yer unfortunate enough to still have one. In fact let them see how scared ye are of her finding out and impress on them that procreation at this point in life is a death sentance. Tell them ye will gleefully give them up fer adoption if they dont toe the line and mean it.
Sons will be too terrified of ever looking at Uncle Charlies magazines ever again, and daughters will be given a guilt complex about ruining daddys life even worse than it already has been just getting to that point. By the time they do start ruining their own lives, they will be old enough to throw out on the street.
Never underestimate psychological warfare with teenagers...let them think its a democracy...does wonders for their self esteem if they think they have a choice. Ye know ye hit the sweet spot if ye have them making a choice that they think ye wont like, when that was actually what ye wanted them to choose in the first place. Act all indignified now and then...let them think they got one over ye. Lull them into a false sense but letting them think yer a pushover...they make more mistakes when they think they better than you and its far easier to catch them out in the end ^