Should you have sex with your high school boyfriend? My answer is simple: No. You should wait. This is not for the reasons that you have likely heard from priests and parents--dictates of morality and virtue. Rather, it is for a reason that should strike you as much more relevant and worthy. I advise you to wait because you will be a happier and healthier adult for having done so.
Please do not write this off as a religious figure telling you what the Bible says is best. On the contrary, my response derives from years and years of speaking with young women who decided to have sex much earlier than they actually wanted to, with men they weren't sure loved them or with whom they weren't sure they were in love. It comes from sitting in on many a tear-ridden epiphany when a girl realized she had slept with her boyfriend for precisely the reason that you offer in your letter--"because he wanted to"--and that this reason had completely eliminated her needs from the equation.
Keep in mind that life is meant to happen in stages. When we skip ahead
to reach a stage that we are not yet ready for, we suffer repercussions.
In your situation, the consequences range from an unwanted pregnancy (at
an age when you should be focused on college and soccer games and the
prom) to the wish that you had your full physical self to give to the
man you will eventually love and share your life with. Another very real
aftermath of engaging in sex before one is ready is the risk that you
will learn not to enjoy it. Sex for sex's sake causes many women to
dread the act. They come to view it as an obligation. Gauging the
tentative language in your letter, I can only deduce that you yourself
are not ready to take this major step.
Physical intimacy will not have a solid foundation until true emotional intimacy has been established. This progression is relevant whether you are 16 or 60. If physical intimacy is attempted before an emotional base is well-established, the chances of a lasting relationship are slim. Do not skip steps. Lay your foundations--in life and in your relationships--and then build up.
I am also a bit troubled by your reference to "waiting until you are 30" for marriage. I hear this statement more frequently every day. I find it genuinely troubling that an entire generation of young people has decided en masse that love and relationships fall way down on their priority list. Sure, they will spend untold hours at work--sacrificing sleep and well-being in order to get ahead--but they will not commit to more than 60 minutes of chitchat at Starbucks when meeting a potential love interest. To simply declare, when you are only 16 years old, that you will not marry until 30 seems to be ruling out all the possibilities for love you may encounter over the next 14 years.
What happens if at only 23 you meet the man you know you wish to spend your life with? Do you say, "I must wait until 30 because that is the resolution I made"? What chances for happiness you will miss if you stick to your "rule"! Besides, deciding that love is a luxury and not a need will close you off to ever finding it. I have seen this happen time and again. If you are not open and willing to accept the feeling when it comes, it will bounce right off you, and a potentially wonderful opportunity for a permanent commitment will be missed.