You probably aren't going to like this reply, but it sounds to me like your parents are just very concerned for you. And if you've only been using condoms for birth control, then they have a bit of a point for worrying that you'll become pregnant. Don't get me wrong -- condoms are wonderful, they're a very good way to avoid STD and STI transmission -- but it's best to combine them with another form of contraception, such as birth control pills. You might also consider going with your boyfriend to get STD tests for the both of you.
Then I'd suggest letting your parents know that you are practising safer sex, that you're being responsible for your health, and that you're aware of the risks involved. If you're not comfortable talking with them, maybe you could write them a letter and give it to them to read, and ask them to reply in a letter of their own if they have questions or concerns about what you've said.
Here are some articles that might be helpful:
Safe, Sound & Sexy ? A Safer Sex How-To
Margaret Sangers Disneyland - An easy chart of your birth control options
Sexual Negotiation for the Long Haul
"The only unnatural sexual act is that which you cannot perform."
-- Alfred Kinsey
[This message has been edited by kythryne (edited 02-10-2002).]