To answer your question, your parents cannot force you to abort. Be aware that, in PA, abortions are (currently) legal to 24 weeks (5.5-6 mos). That said... (sorry, non-legal stuff coming, but important stuff anyway...)
You need to think seriously about how you see this playing out. Marrying the young man is out of the question. PA law requires both of you to be 18 - no exceptions. Realistically speaking, the young man is unlikely to stick by your side. Yes, it's possible, but 16yo boys aren't always the most... monogamous of creatures.
So, where does that leave you? A 15yo girl, with a heavy responsibility - both to yourself and to your child. While PA doesn't generally allow dropping out until 16 (w/your parents' consent), having a child is an exception that bypasses that to allow it earlier (w/consent). Bad idea, to be honest. There are few jobs you could get at your age that would provide what you need to support your child. While your parents are required to provide necessities for you, they aren't required to help you care for your child - they don't have to provide daycare while you go to school or work, they don't have to babysit so you can go out with your friends or just want a break, they don't have to get up to feed/change the baby at night, etc. That's all on you.
Any hopes you may have of "life" after high school (assuming you graduate) will radically change. Going away to college will be out. Work or work/community college will be about it. Social life? Assuming your current b/f doesn't stay, few 18/19/20 yo men are overly interested in young women who have the responsibility of a child. A lot of your girlfriends will drift away, as well. You all will simply no longer have much in common, not to mention you won't really have much time, nor the resources to pay for frequent sitters.
Alternatively, abortion is an option. So is adoption, as cbg mentioned. That's not an easy decision to make. And, IMO, one that should be considered seriously. Adoption options are greater today than they were in the past. Open adoptions are more common than they used to be and can be framed in many different ways, based on both your and the adoptive parents' wishes. You also have more control regarding who adopts the child. It can truly be a wonderful gift - to both the couple who opens their home and to the child.
As an example, my daughter and son-in-law just recently adopted an infant, via an open adoption. The birth-mom chose them, based on their application, a video they prepared, and an in-person meeting. AFAIK, the agreement provides for a minimum of three visits/year (as birth-mom desires) and monthly photos/updates.
Take some time to think - seriously think - about what is best for you at this stage of your life, as well as (if you choose to keep the pregnancy) what is best for the child. While you are still a child (yes, you are, stop rolling your eyes at me, young lady!), you are in an adult situation and need to make an adult decision. Give it the seriousness and maturity it deserves.
Best of luck.
