Ten year olds are certainly curious about sex from their own body changes and of course the media.
Please please read the book Sex & Sensibility: The Thinking Parent's Guide to Talking Sense About Sex by Baltimore/Washington specialist, author, educator, etc, Deborah Roffman, known as the "sex lady" to many independent schools. Her emphasis is on how to integrate information about sexuality from birth throughout life. If the vocabulary is already part of your family conversation, it's so much easier to talk about the "specifics". And don't assume this issue will no longer be hanging around after you have the "big talk"--this is an evolving development in your parenting life.
If you are so uncomfortable about the subject, say so. You might also ask your PTA to bring in a specialist such as Roffman to talk with your community. Roffman also talks to students at many schools.
AND, it is so much easier to talk straight about sexual development when your daughter/son is 10 and you are certain your child is not engaging in sex than when she is 14 and already "playing around". That can raise the concerns about whether the "talk" is a way of giving permission to indulge.
Remember that values are just as important as "hard, cold facts". It is a formidable challenge when helping children sort through the hash of family values, personal beliefs, media messages, and peer pressure about sex. Our children need to know the characteristics of a healthy sexual experience as well as an abusive one. Girls especially need to appreciate mutually in a sexual relationship--more relevant in light of over-reported oral sex among middle school kids. A core message to both our sons and daughters is that sexual pleasure is an activity this is shared equally between partners.
By encouraging more age and maturity before considering sexual activity gives our children a better shot at making responsible decisions about when and with whom as well as handling the intense emotions that come with intimacy (including rejection and disappointment). Lastly, some child psychologists find that girls get more pressure from their female peers than from boys to experiment sexually.
D. Hart, Ed.D., www.edpathways.net